Added function for reading RSSI from a analog singal (with RC-Filter)

Hello,

added function for reading RSSI from a analog singal (RC-Filter)

New Master-Config:
```c
- uint8_t rssi_adc_channel; // Read analog-rssi from RC-filter (RSSI-PWM to RSSI-Analog), RC_CH2 (unused when in CPPM mode, = 1), RC_CH8 (last channel in PWM mode, = 9), 0 to disable (disabled if rssi_aux_channel > 0 or rssi_adc_channel == power_adc_channel)
- uint16_t rssi_adc_max;	// max input voltage defined by RC-filter (is RSSI never 100% reduce the value) (1...4095)
- uint16_t rssi_adc_offset; // input offset defined by RC-filter (0...4095)
````
Tested with Frsky D4R-II

Example for a RC-filter:

     PWM-RSSI   ___
     D4R-II o--|___|-----o-----o RC_CH2/RC_CH8 NAZE32
                         |
                R1 10K   |
                        ---  C1
                        ---  47µF
                         |

rssi_adc_max = 2950
rssi_adc_offset = 160


----
#162 closed (sry my mistake)
technically it's not needed to initialize channels to zero since the whole structure is already cleared with default values. there's some leftovers of that in the code, but I've been slowly removing them.