binman: Add support for adding a name prefix to entries

Sometimes we have several sections which repeat the same entries (e.g. for
a read-only and read-write version of the same section). It is useful to
be able to tell these entries apart by name.

Add a new 'name-prefix' property for sections, which causes all entries
within that section to have a given name prefix.

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