cmake: dts: look for allowed dts overlays file presence in board/

Hardware model v2 allows the usage of <board>_<revision>.overlay and any
valid <board>_<identifier>_<revision>.overlay dts overlays in the board/
directory but CMake was only looking for the most specific
<board>_<identifier>_<revision>.overlay overlay file.

This commit allows CMake to look for the presence of any allowed overlay
file to fix this issue.

Tested by building the hello_world sample with the Olimex board
lora_stm32wl_devkit revision C.

The following line wasn't appearing before in build log and now does:
"-- Found devicetree overlay: <path>/olimex_lora_stm32wl_devkit_C.overlay"

Signed-off-by: Johan Lafon <johan.lafon@syslinbit.com>