cmake: mcuboot: refactor the checks for west and the key files

As of c952f09a the calls to west
sign were replaced with imgtool but a lingering integration with
WEST_TOPDIR was allowed to remain which is not needed when there
are absolute paths available for the configuration. So, this
attempts to refactor the code to allow a few things

- allow a search of relative paths to application config and
  then west topdir
- only fatal error when a west workspace is needed but not found,
  so if the config is all absolute files then a west workspace is
  not required

Fixes: #86438
Signed-off-by: Charles Hardin <ckhardin@gmail.com>