[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>