FrSky telemetry update

Several change for FrSky telemetry :

- frsky_default_lat / frsky_default_lon : leave it to 0 if you don't want to display compass information in telemetry if you don't have GPS fix, otherwise set the value you want
- frsky_coordinates_format : 0 for DMS (default) or 1 for NMEA
- frsky_unit : 0 for METRICS (default) or 1 for IMPERIALS, this one is used for temperature2 (Num satellite / hdop)
- frsky_battery_size : leave it to 0 will display consumption, set it to a different value (e.g. 1300 for 1300 mah), it will compute the remaining percent.

RPM will display battery size (not armed), or throttle pulse (armed). For this to work, you need to set blade number to 12 in Taranis.

One change not for telemetry :
 - gps_auto_config : it's not related to telemetry, it's just a parameter to allow dynamic configuration (0, default value) or static configuration for ublox gps (1). I prefere to have my own ublox configuration, based on this one (https://raw.githubusercontent.com/diydrones/ardupilot/master/libraries/AP_GPS/config/3DR-Ublox.txt)