KK2.0 v1.6++ 
Features
This custom firmware was written by me for the KK2.0 controller and it has the following new features:

- This version is for traditional receivers only. CPPM and S-Bus versions will be released soon (with additional features).
- The 'SL stick activation' feature was removed so aileron input can no longer be used to turn 'Self-level' mode on and off.
- It has a completely new 'SL stick mixing' mode with four settings (Off, Low, Medium and High) selectable from the 'Mode Settings' screen. This will give a soft transition from Self-level to ACRO mode based on aileron/elevator stick deflection. The selected setting will be displayed on the SAFE screen as 'ON' (for original SL mode), 'ON MIX L', 'ON MIX M' or 'ON MIX H' when your AUX switch is in ON position.
- ESC calibration is now done without a transmitter and it is sufficient to hold down a single button. This will help users with slow binding receivers do ESC calibration in a simple and safe way. Read the instructions below to learn the new ESC calibration routine.
- Alarm (i.e. 'Lost Model Alarm') can be activated from the AUX switch. 'Receiver Test' will display 'Off', 'Alarm' or 'SL On' for AUX. You'll need a 3-way switch assigned to your AUX channel for this to work.
- Support for gimbals with differential mixing (in addition to normal gimbals). This is configurable from the 'Camera Stab Settings' screen. Borrowed from Brontide's firmware.
- A 'Factory Reset' will clear the mixer table instead of loading the QUAD+ motor layout. This feature is borrowed from Brontide's firmware and further improved by me. An error message (i.e. "Error: Motor Layout!") is displayed on the SAFE screen and arming is refused until a motor layout has been loaded.
- Gyro limits have been expanded to let boards with slightly damaged gyros arm. Values borrowed from Steveis' firmware.
- The 'Version Info' screen is now accessible from the main menu and is not displayed during start-up anymore. Start-up time is reduced by one second thanks to this change.
- Made a few changes to the GUI. Hope you'll like them. 
- It is based on the original KK2.0 1V6 firmware and therefore includes the same improved acro handling as in my previous KK2.0 ACRO patch.

I have tested this firmware on my own RC models and it works without problems for me, but please read the warning below to understand the risks of using this firmware on a damaged KK2.0 board. I would hate to see anybody get hurt by using this firmware, so please take care.

WARNING!
DO NOT USE THIS FIRMWARE IF YOUR KK2.0 BOARD SUFFERS FROM DAMAGED BUTTONS OR IF YOUR ESCs HAVE PROBLEMS ENTERING CALIBRATION MODE!
IF A SINGLE BUTTON IS STUCK IN 'DOWN' POSITION AND YOUR ESCs FAIL TO ENTER CALIBRATION MODE, YOUR MOTORS WILL GO FULL THROTTLE AS SOON AS YOU PLUG IN YOUR FLIGHT BATTERY AND THEY CANNOT BE SHUT DOWN FROM YOUR TRANSMITTER! 

REMEMBER: YOU USE THIS FIRMWARE AT YOUR OWN RISK!

Yes, that was a serous warning, but you should also know that it is perfectly safe to use this firmware as long as all KK2.0 buttons are working properly and your ESCs have no problems entering calibration mode. Don't be afraid to test the new ESC calibration routine with propellers off.
Now that you have read and understood the warning above, we can go on with the new and simplified ESC calibration routine. 

ESC calibration routine (no transmitter required):
1. Remove props, but leave the motors connected so that you can hear the ESC confirmation beeps!
2. Disconnect servos if they cannot handle a sudden change from endpoint to endpoint.
3. Hold down any* button (i.e. a single button), except for the leftmost one, while connecting the flight battery.
4. Keep holding the button down while waiting for the 'High throttle' level beep(s) and then release it to set the 'Low throttle' level.
5. Push button 1 (labeled 'EXIT') after the final ESC confirmation beeps to return to the 'SAFE' screen.

* If you hold down button 1 while connecting the battery you'll see a warning (and probably hear some 'angry' beeps). Just disconnect the battery and try again (holding down a different button this time).

This calibration method will set the upper throttle level to 2.0 ms and the lower throttle level to 1.02 ms, which are the exact same values used by the original KK2.0 1V6 firmware (Don't be fooled by the "Throttle pass-through" message).

I actually recommend testing the new ESC calibration routine (WITH PROPELLERS OFF) at least once to see if your ESCs have problems entering calibration mode. Do not use this firmware if your motors spin up!

You will probably notice that the 'Version Information' screen and its one second delay has been removed from the start-up sequence. This will make the ESC calibration routine safer because your ESCs will now see the 'full throttle' signal one second sooner (compared to the original KK2.0 1V6 firmware) and this reduces the risk for having motors going full throttle while doing the ESC calibration.

SL Stick Mixing
The new 'SL Stick Mixing' mode is activated from the 'Mode Settings' screen where you can set three levels for 'SL P-gain' reduction rate or you can turn it off completely (default) to use the original Self-level mode. You select the wanted setting by pushing button 1 - 4 on the new screen that shows up when you select 'CHANGE' on the 'SL Stick Mixing' item.
The reduction rate (LOW, MEDIUM or HIGH) should be selected based on your P-gain setting on the 'Self-level Settings' screen. If your SL P-gain setting is below 30 you should select LOW for reduction rate. I would recommend selecting the MEDIUM reduction rate for SL P-gains in the 30 to 60 range and HIGH for gain values above 60 and up to 100. It is not recommended to use this feature for SL P-gain values above 100.
I have tested this mode myself and I like it very much. I have even performed a full throttle loop without problems. If you get in trouble (e.g. lose orientation) you can just center the aileron/elevator stick and the model will level itself. Another advantage is that you don't have to ramp up your 'Stick Scaling' values for flying around (compared to the original Self-level mode in KK2.0 1V6).

Observe:
- You should tune your model as best as you can in both acro and SL mode before trying the new 'SL Stick Mixing' mode.
- Don't count on this mode saving your model if you perform acrobatic stunts and exceed the 440 degree/second gyro limitation. It may actually make it worse! 
- If your model has built up any momentum, it will not stop moving immediately after centering the aileron/elevator stick! Take it slow if you're at beginner level.
- This mode will work best for transmitters configured to use mode 2 or 3 (i.e. with aileron and elevator on the same stick).

Enjoy! 
Please read the information above (and especially the warning) before downloading this firmware.
Remember to write down your settings, for this firmware will certainly erase them all!