board: arm: Add Support For 96Boards Aerocore2

Changes:

- Added all required board files in /boards/arm/96b_aerocore2
- Modified pinmux for stm32f4

Most of the changes in this PR is based on reverse-engineering of the
PCB layout and following commits in the PX4 firmware repository for
the same board. The manufacturer does not provide and or generate
schematics and pinout tables for this board.

This PR includes almost all of the interfaces connected to the STM32
MCU, the only thing not included is the J9 and J8 headers that connect
to a 96Boards baseboard.
These headers are not vital to the functionality of the Aerocore2.

Signed-off-by: Sahaj Sarup <sahaj.sarup@linaro.org>