binman: Remove '/images/' fragment from FIT subentry paths

binman: Remove '/images/' fragment from FIT subentry paths

Binman FIT entry nodes describe their subentries in an 'images' subnode,
same as how they would be written for the mkimage executable. The entry
type initially manually managed its subentries keyed by their node paths
relative to its base node. It was later converted to a proper section
while still keeping the same keys for subentries.

These subentry keys of sections are used as path fragments, so they must
not contain the path separator character '/'. Otherwise, they won't be
addressable by binman extract/replace commands. Change these keys from
the '/images/foo' forms to the subentry node names. Extend the simple
FIT tests to check for this.

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>