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
HAL_PX4: writing more than 512 bytes at a time can cause IO errors

the vfat fs in NuttX is really very picky!
HAL_PX4: format panic messages with a newline
AP_PerfMon: fixes to make it work under AP_HAL (almost)
Copter: inav loiter comment changes

removed unused get_loiter_vel function
HAL_PX4: handle IO errors on microsd

this should handle EINTR gracefully
HAL_PX4: added UART performance counters
HAL_PX4: added storage performance counters
HAL_PX4: added scheduler performance counters