Add Gumstix AeroCore device

Based on the work of Andrew Smith [1], add board configuration to
support the Gumstix AeroCore (previously Aerodroid) board [2]. This
is an autopilot board based on a STM32F427 similar to the FMUv2.

[1] https://github.com/smithandrewc/Bootloader
[2] https://store.gumstix.com/index.php/products/585/

Signed-off-by: Ash Charles <ashcharles@gmail.com>