Improved CPPM - alternative GPIOs, arming and altitude hold, init fix

[matejkarasek]

This PR further improves the support of external CPPM receivers:

• alternative GPIO's can be used: PA7(MOSI), PA2(TX2), PA3(RX2), PB4(IO_3), PB5(IO_2) or PB8(IO_1). This allows to use a CPPM receiver alongside many of the decks without pin interference
• switches can be used for arming and triggering altitude hold
• initialization of all channels is improved to prevent undefined behaviour when no physical receiver is connected, or when transmitter is switched on after the crazyflie
• logs are split into two groups: ext_raw for raw channel data (8 channels now) and ext for processed data
• some other minor fixes

[matejkarasek]

Flight tested on a CF2.1 with FrSky R-XSR receiver

[tobbeanton]

I've tested this with the default PA2 pin and it works well. I have not tested with the others but anyway think this pull request is ok to merge. Well done and many thanks!

[matejkarasek]

Great, and thanks for testing it and the merge!