External ADC pad support (Rev5 board) for rssi and power meter

Added use of "ADC" pad on bottom of Rev5 boards with RSSI and POWER METER. Setting power_adc_channel or rssi_adc_channel to 5 allows this.
defaulting to 0 isn't needed as the struct is memset()'d anyway.
there are some variables left over as being set to 0 but its not necessary.