ci/bare-metal: introduce BM_MKBOOT_PARAMS

Make it possible to provide per device mkimage.py params.

Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Reviewed-by: David Heidelberg <david.heidelberg@collabora.com>
Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13691>

.gitlab-ci/bare-metal/fastboot.sh

@@ -120,6 +120,7 @@ mkbootimg.py \
   --ramdisk rootfs.cpio.gz \
   --dtb dtb \
   --cmdline "$BM_CMDLINE" \
+  $BM_MKBOOT_PARAMS \
   --header_version 2 \
   -o artifacts/fastboot.img