cmake: fix DTS handling of multiple overlay files
cmake: fix DTS handling of multiple overlay files

This commit fixes an issue where providing a list of overlay files for
dts was not handled correctly.
The means that using `-DDTC_OVERLAY_FILE="file1.overlay;file2.overlay"`
would fail as those two file names would be concatenated into a single
file name.

This is fixed by properly quote the `DTC_OVERLAY_FILE` variable.

Signed-off-by: Torsten Rasmussen <a7c5b1d5a1f41fce6edfa3f7c2392ae0c59deafe@nordicsemi.no>

finikorg/zephyr,finikorg/zephyr,Vudentz/zephyr,Vudentz/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,nashif/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,Vudentz/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,Vudentz/zephyr,nashif/zephyr