binman: Support simple templates

Collections can used to collect the contents of other entries into a
single entry, but they result in a single entry, with the original entries
'left behind' in their old place.

It is useful to be able to specific a set of entries ones and have it used
in multiple images, or parts of an image.

Implement this mechanism.

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