MSP protocol support test branch version 0.7

Switched to MSP command mode (we respond to polling as default)

Supported OSD widgets:

MSP_OSD_FMODE shows !FS! when failsafe is triggered (battery, radioi, gcs, ekf)
MSP_OSD_VCELL average cell voltage (cell count override MSP_OSD_NCELLS 0:AUTO, 1-N)
MSP_OSD VBAT battery voltage
MSP_OSD_MAH battery consumed mAh
MSP_OSD_BATT battery usage bar %
MSP_OSD_CURR battery current
MSP_OSD_SATS gps sat count (shows 14 sats when there's no fix it's a known DJI issue)
MSP_OSD_LAT, MSP_OSD_LON gps coordinates
MSP_OSD_HDIST home distance
MSP_OSD_HDIR home direction
MSP_OSD_ALT altitude
MSP_OSD_MSG text area with flight mode shared with status text messages and wind
MSP_OSD_ROLL roll
MSP_OSD_PITCH pitch
MSP_OSD_RSSI rssi
MSP_OSD_ARM arming status
MSP_OSD_RTC date and time (RTC)
MSP_OSD_PWR power in Watts
MSP_OSD_VSPD numerical vertical speed
MSP_OSD_GSPD ground speed
MSP_OSD_ASPD_EN airspeed as override of ground speeed
MSP_OSD_WIND_EN wind direction and speed (rendered as text and arrow next to the flightmode)
MSP_OSD_THR throttle channel (experimental)
MSP_OSD_TESC blheli ESC temperature
MSP_OSD_UNITS 0:Imperial, 1:Metric
Artificial horizon:

MSP_OSD_HBARS ladders
MSP_OSD_CROSS crosshair
MSP_OSD_HORIZ horizon line (does not work, DJI bug?)