Copter: reenable optical flow by default

correct optflow test compile error
Copter: remove unnecessary Log_Write_Optflow
HAL_SITL: get the ublox GPS timestamps right in SITL
AP_GPS: added support for GPS time in week/millisec

also adds time_epoch_usec() for MAVLink SYSTEM_TIME
HAL_SITL: properly emulate timestamps on ublox and MTK
AP_InertialNav: updates for new GPS API
DataFlash: updates for new GPS API
Plane: added MAVLink SYSTEM_TIME message
Copter: added MAVLink SYSTEM_TIME message
Rover: added MAVLink SYSTEM_TIME message
AP_GPS: added fake time to fake GPS lock
AP_HAL: added set_system_clock() API
HAL_PX4: implement set_system_clock()
DataFlash: show timestamps on flash logs
Plane: set system time on GPS lock
Rover: set system time on GPS lock
Copter: set system time on GPS lock
HAL_SITL: fixed time strings in simulated NMEA GPS
AP_GPS: fixed date handling in NMEA driver
AntennaTracker: first cut at antenna tracking sketch