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>
[l3gd20] Add support for L3G4200D chip

The L3G4200D chip is very similar to the L3GD20[H] parts and can use
the same driver with minor adjustments.  There are four differences:
 * WHO_AM_I register is 0xD3 (not 0xD4 or 0xD7):
   - added an extra case to the driver probe
 * Sampling rates are marginally different:
   - setting sampling rate now depends on the detected chip
 * I2C address range is different:
   - no changes as the driver doesn't support i2c access
 * the L3G4200D has a self-test function:
   - no changes---chose not to implement feature in driver

Signed-off-by: Ash Charles <ashcharles@gmail.com>
[gps] Conditionally set default GPS port

AeroCore uses ttyS0 not ttyS3 as the serial port connected to the GPS.
Now, a board can set GPS_DEFAULT_UART_PORT to override the default setting
in a board-specific fashion.

Signed-off-by: Ash Charles <ashcharles@gmail.com>
[l3gd20] Style fixes for l3g4200d integration

As requested here [1], this fixes some of the style errors
introduced with the addition of l3g4200d support to the l3gd20
driver.  Gyro orientation is set conditionally for the aerocore
board.

[1] https://github.com/PX4/Firmware/pull/937

Signed-off-by: Ash Charles <ashcharles@gmail.com>
[aerocore] Remove commented code for GPIO2

GPIO2 is currently used for the tone alarm, not an arbitrary GPIO.

Signed-off-by: Ash Charles <ashcharles@gmail.com>
[l3gd20] Make gyro orientation board-overridable

As discussed [1], provide a default SENSOR_BOARD_ROTATION
(270 degrees as this seems most common) and let boards override it
as necessary.

[1] https://github.com/gumstix/m4-firmware/commit/7d0850a710b3ac9e9e165beb36389577d0e5adcb#commitcomment-6315550

Signed-off-by: Ash Charles <ashcharles@gmail.com>
Merge pull request #937 from gumstix/aerocore

Aerocore