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)
constrain is a normal function 

```c 
int constrain(int amt, int low, int high)
{
    if (amt < low)
        return low;
    else if (amt > high)
        return high;
    else
        return amt;
}
```