RT_CFG_Update_Start_Marker_2: Update start marker position
RT_CFG_Test: 159, 18 (0x9F1220 + unit ID) (0x7D2)
Bytes 0-3: Start Marker Latitude (degrees)
Bytes 4-7: Start Marker Longitude (degrees)
Latitude resolution is degrees * 1e-7.
Longitude resolution is degrees * 1e-7.