binman: Allow collection to use entries from other sections

At present the collections etype only works with entries in the same
section. This can be limiting, since in some cases the data may be inside
a subsection, e.g. if there are alignment constraints.

Add a function to find the entries in an etype and have it search
recursively. Make use of this for mkimage also.

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