mkimage: Refactor imagetool_get_source_date to take command name

So we can use imagetool_get_source_date() from callers who do not have
the image tool params struct, just pass in the command name for the error
message.

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