SITL: added SIM_GPS_TYPE control

for choosing what sort of GPS to simulate
SITL: added simulation of the original MTK GPS
SITL: added support for MTK16 and MTK19 simulated GPS types
SITL: improve time field handling in simulated GPS
SITL: added SIM_GPS_BYTELOSS option

this allows testing of protocol recovery after losing bytes on the GPS
serial link