Add Gumstix AeroCore device

Based on the work of Andrew Smith [1], add board configuration and device
drivers to support the Gumstix AeroCore (previously Aerodroid) board [2]. The
AeroCore is an autopilot board based on a STM32F427 similar to the FMUv2.

[1] https://github.com/smithandrewc/Firmware
[2] https://store.gumstix.com/index.php/products/585/

Signed-off-by: Ash Charles <ashcharles@gmail.com>

Conflicts:
	src/drivers/px4fmu/fmu.cpp
	src/modules/commander/commander_helper.cpp