sepolicy : add selable for /firmware/image

as there are some meta's packages which using /firmware/image for
firmware image adding it to the list.
where one example was spdaemon was not able to load images with
following permission issue
 spdaemon: type=1400 audit(0.0:668): avc: denied { getattr }
   for path="/firmware/image/crypt_t.sig" dev="sde5" ino=348
    scontext=u:r:spdaemon:s0 tcontext=u:object_r:vfat:s0 tclass=file permissive=1

Change-Id: I335be4a589aaa462ced71134f39f561b4ca7e3c3