External ADC pad support for rssi and power meter

Allowing use of ADC_CHANNEL5 for rssi (rssi_adc_channel) and power meter
(power_adc_channel) on external pad (Rev5 only)
Missed correct number of places power adc comment
Removed unnecessary defaulting to 0

Defaulting to 0 isn't needed as the struct is memset()'d anyway.
Uncommitted (possibly) baseflight.hex

*hopefully* uncommitted baseflight.hex
Merge pull request #174 from geebles/master

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