Collect and use RSSI value.

This adds 3 new fields to ManualControlSettings:
RssiType - Source type of the RSSI info (PWM,PPM,ADC)
RssiChannelNumber - Source channel of the RSSI info.
RssiMax - Max RSSI value, used for scaling.
RssiMin - Min RSSI value, used for scaling.

2 new fields to ManualControlCommand:
Rssi - Scaled (to 0-100) RSSI value based on min and max settings.
RawRssi - raw RSSI value as read from the type driver (PWM,PPM,ADC), can be used for testing to find proper min and max values.

Feeds RSSI value to mavlink stream on UAVOMavlinkBridge.