There are a small handful of samples that still utilize <board.h>
include, to minimize the use of <board.h> to these specific cases and
allow us to remove adding the board dir to the top level include search
path, we explicitly in each sample add the specific board dir it needs.
For the microbit cases these could be replaced by DTS support in the
future when the pwm_nrf5_sw supports DTS.
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>
