mkimage: fit_image: Add support for SOURCE_DATE_EPOCH in signatures

When generating timestamps in signatures, use imagetool_get_source_date()
so we can be overridden by SOURCE_DATE_EPOCH to generate reproducible
images.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Reviewed-by: Simon Glass <sjg@chromum.org>
3 files changed