AP_Compass: use report timestamp on PX4 for accurate timing
AP_Baro: fixed timestamp on baro for PX4

milliseconds not microseconds
SITL: allow motor multiplier to work on quad simulation
HAL_PX4: rewrite UART driver to be much more efficient

this does the IO in the timer thread, and uses buffers in the main
task to avoid system call costs in the flight code.

The cost of PX4 read and write system calls is quite high - about 10
to 15 usec. We can't afford to pay that per byte