image: Fix "metadata is not deterministic" when chaining 2+ CONVERSION_CMDs
When we have more than one CONVERSION_CMD being used, for example
ext4.gz.sha256sum we will see errors about "metadata is not
deterministic". This is because we do not have a stable order of
intermediate files that will be removed in the generated shell command.
We fix this by calling sorted() on the set of rm_tmp_images so that we
will have a stable hash again.
Cc: Patrick Ohly <patrick.ohly@intel.com> Signed-off-by: Tom Rini <trini@konsulko.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
