grub2: Add GRUB_MKIMAGE_OPTS for configurable grub-mkimage options

grub2: Add GRUB_MKIMAGE_OPTS for configurable grub-mkimage options

Introduce the GRUB_MKIMAGE_OPTS variable to allow additional options
to be passed to grub-mkimage, making its behavior more configurable.
(for example, this allows disabling shim lock when necessary).

Update do_mkimage to use ${GRUB_MKIMAGE_OPTS}. This change reduce the
need for hardcoded modifications and makes future adjustments easier.

Signed-off-by: Valeria Petrov <valeria.petrov@spinetix.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>