An invalid value in IMAGE_FSTYPES was triggering a traceback. Add a
check and a reasonable error message instead.
(From OE-Core rev: f38c35b925d6eab0d5a41b8592b0eb8b0a3b7b72)
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>