cmake: Fix doubling FILE_SUFFIX on boards/*.overlay files

When building with `-DBOARD=plank -DFILE_SUFFIX=foo`, the build system
could set `DTC_OVERLAY_FILE` to one of:

  - boards/plank_foo_foo.overlay
  - boards/plank_foo.overlay
  - boards/plank.overlay

The first choice is a bug, as the suffix should only be applied once.

Signed-off-by: Grzegorz Swiderski <grzegorz.swiderski@nordicsemi.no>