The struct image has "uuid" and "size" to memorize the field values
they had in the TOC entry.  So, parse_fip() copies them from struct
fip_toc_entry to struct image, then pack_images() copies them back
to struct fip_toc_entry.

The next commit (support --align option) will require to save the
"offset" field as well.  This makes me realize that struct image
can embed struct fip_toc_entry.

This commit will allow the "flags" field to persevere the "update"
command.  At this moment, the "flags" is not used in a useful way.
(Yet, platforms can save their own parameters in the flags field.)
It makes sense to save it unless users explicitly replace the image.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>