RSSI_ADC configurable scale in CLI

A configurable scale for RSSI (rssi_scale in CLI) is applied to the rssi voltage for tuning. Set in CLI its value is between 1 (to avoid a 0 division) and 255.
The rssi voltage (applied on RC2 of NAZE32) is obtained with a low pass RC filter (R=220K and C=1microF) on the PPM signal. For my configuration, the resulting voltage is 2V which then can be set to a 96% RSSI level (with rssi_scale=29).