binman: Allow reading entries from a subnode

binman: Allow reading entries from a subnode

Some images may have multiple copies of the same thing, e.g. two versions
of the read/write U-Boots. It is necessary to read data from one or other
of these under selection of the verified-boot logic. Add a function to
select the subnode to use.

Signed-off-by: Simon Glass <sjg@chromium.org>