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()
Rover: use common available_memory()