Initial Battery Monitoring/dynamic OSD/dynamic ADC coding

OSD:
- added eeprom/cli for displaying altitude hold state
- added eeprom/cli for changing altitude/heading location on screen

battery monitor
- added eeprom/cli for changing pins, scale, bias etc. for voltage and
current

adc
- added "adcValue(uint8_t pin)" function to read from any ADC 4-6 (or
onboard voltage divider). See pin defines at top of drv_adc.c

Minor tweaks here and there to keep things consistent

to do:
- add voltage/current display to OSD
- test ADC and battery monitoring code, OSD code
- add RSSI reading/eeprom/cli and OSD widgets
- add callsign display to OSD, eeprom/CLI
- add compass bar to OSD, eeprom/cli for enabling/disabling/position
- …?

would be cool but probably won't get to for a while
- display location of all OSD stuff in CLI
- autoranging RSSI (read max values, store, read min values, store,
calculate scale/bias).

Will continue working on soon, none of this has been tested or even
compiled so it's possible I broke everything...