binman: allow user-defined filenames for mkimage entry

mkimage entry currently creates a file whose name is derived from the
section name containing said entry.

Let's allow the user to define a filename for the mkimage-generated
binary by using the 'filename' DT property.

Cc: Quentin Schulz <foss+uboot@0leil.net>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>