cmake: dts: Allow DTS_BOARD_FIXUP_FILE to be defined anywhere
cmake: dts: Allow DTS_BOARD_FIXUP_FILE to be defined anywhere

Allow for DTS_BOARD_FIXUP_FILE to be defined in other locations (ie by
board directory code).  Thus allow the board to set any location to find
the fixup file.

Signed-off-by: Kumar Gala <a5e5248af4cd4f0ed8c515f61d40a6e2db46a66e@linaro.org>

zephyrproject-rtos/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,zephyriot/zephyr,ldts/zephyr,ldts/zephyr,Vudentz/zephyr,nashif/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,aceofall/zephyr-iotos,explora26/zephyr,galak/zephyr,galak/zephyr,explora26/zephyr,galak/zephyr,mbolivar/zephyr,mbolivar/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,mbolivar/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,aceofall/zephyr-iotos,mbolivar/zephyr,nashif/zephyr,finikorg/zephyr,explora26/zephyr,mbolivar/zephyr,zephyriot/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,aceofall/zephyr-iotos,kraj/zephyr,ldts/zephyr,punitvara/zephyr,Vudentz/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,kraj/zephyr,aceofall/zephyr-iotos,GiulianoFranchetto/zephyr,zephyriot/zephyr,explora26/zephyr,zephyriot/zephyr,aceofall/zephyr-iotos,GiulianoFranchetto/zephyr,finikorg/zephyr,kraj/zephyr,nashif/zephyr,zephyriot/zephyr,Vudentz/zephyr,Vudentz/zephyr,nashif/zephyr,finikorg/zephyr,punitvara/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,punitvara/zephyr