cmake: Fix bug where dts.overlay was not being appended
cmake: Fix bug where dts.overlay was not being appended

Fix bug #399 where dts.overlay was not being appended.

The build scripts were erroneously modifying the shadowed non-cache
variable instead of the cache variable.

Signed-off-by: Sebastian Bøe <edee888afd63e94916688495334700803eba20a0@nordicsemi.no>

ATmobica/mcuboot,tamban01/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,utzig/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,utzig/mcuboot,tamban01/mcuboot,ATmobica/mcuboot,tamban01/mcuboot,tamban01/mcuboot,utzig/mcuboot,utzig/mcuboot,runtimeco/mcuboot,tamban01/mcuboot,runtimeco/mcuboot,utzig/mcuboot