InertialNav: Fixed signs, remove body-frame rotation, apply correction at 100hz.
InertialNav: added 300ms timeout after which error from gps heading will be set to zero
Copter: Leonard Hall's inertial nav based loiter
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
HAL_PX4: fixed blocking IO in UART driver
HAL_PX4: avoid RCOutput ioctl unless values change

this lowers latency somewhat
HAL_PX4: use pthread_yield() in main loop

this lowers the latency of the main loop somewhat
HAL_PX4: fixed rcin->valid() for RC override input
HAL_PX4: run timer thread as SCHED_FIFO

this ensures it never yields to the main thread, which would break our
locking assumptions
HAL_PX4: ensure all writes go via buffers

this prevents output corruption by direct fd writes
Copter: limit number of bytes read per GCS check
Plane: limit number of bytes read per GCS check
Plane: fixed typo
Copter: Leonard Hall's inertial nav ver 2
Copter: typo fixed for trad heli config
HAL_PX4: new buffered storeage driver for microsd cards

this does all IO in the timer thread, avoids writes that don't change
data, and does all writes in multiples of 128 byte chunks. This should
be about as friendly to SD cards as we can get.
Copter: add "f" to end of float constants

This is a small performance improvement for the px4