Plane: initialise scheduler callback after logging

this could cause log corruption otherwise
Rover: initialise scheduler callback after logging

prevents possible log corruption
INS: replace APM with vehicle in accel calibrate
Copter: replace APM with board in cli
Rover: replace APM with board in cli
Plane: replace APM with board in cli
Plane: break up GPS and logging, allow GPS update at 50Hz

this prevents mismatches in GPS message arrival and accel message
arrival from causing small DCM errors
HAL_PX4: use larger buffers on USB
DataFlash: stop write logging while reading

prevents nuttx getting confused
GCS_MAVLink: sped up downloading DF logs over MAVLink
HAL_AVR: moved memcheck into HAL_AVR
AP_HAL: added available_memory() interface
HAL_AVR: use memcheck for available_memory()
Plane: use AP_HAL available_memory() call
GCS_MAVLink: moved send_meminfo() to GCS_Common.cpp
GCS_MAVLink: pushed num_sends higher for faster USB log transfer

up to 169 kbyte/sec on pixhawk
HAL_PX4: implement available_memory()
HAL_PX4: even larger tx buffer size for faster log transfer
Plane: use common send_meminfo()
Copter: use common available_memory()