binman: Include also subnodes in generator nodes

This allows to prefill fdt and config nodes with hash and signature
subnodes. It's just important to place the child nodes last so that
hashes do not come before the data - would be disliked by mkimage.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: Simon Glass <sjg@chromium.org>