Make kconfigfunctions.py agnostic to where GENERATED_DTS_BOARD_CONF is
located.
We don't want to encode the path of GENERATED_DTS_BOARD_CONF into more
places than necessary. Also, re-using GENERATED_DTS_BOARD_CONF makes
it easier to change where generated_dts_board.conf is located.
Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>