[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>