HAL_PX4: support uartE for 2nd GPS
AP_HAL: support uartE
HAL_AVR: no uartE
HAL_Empty: no uartE
HAL_FLYMAPLE: no uartE
HAL_SITL: support a 2nd GPS
DataFlash: added GPS2 logging
Plane: support a 2nd GPS
Rover: support a 2nd GPS
Copter: added support for 2nd GPS
AP_Motors: removed unused variable
AP_AHRS: remove unused variable
AP_HAL: allow for microsecond resolution in scheduler->stop_clock()

this makes 400Hz in the replay code possible
AP_NavEKF: support 400Hz replay from 50Hz logs
AP_InertialSensor: make HIL timing more accurate

needed for 400Hz replay
AP_NavEKF: improved replay timing
AP_NavEKF : Fix variance constraint bug

Constraining variances to a minimum value of 1e-9 was causing problems
with gyro bias and angular accuracy in noisy GPS environments.
Because the constraint is applied after every covariance prediction
and correction, a lower value of 0 is more appropriate.
AP_NavEKF : Improved heading and magnetic field state initialisation
Rover: disable GPS2 in HIL
Copter: disable GPS2 in HIL