scripts: ubinize-image.sh: support static volumes, make size optional

scripts: ubinize-image.sh: support static volumes, make size optional

In order to support devices having TF-A FIP image or UBI-aware U-Boot
SPL we need to include a static volume for the bootloader.

Introduce support for adding additional static volumes by prefixing
the filename with ':', eg.

UBINIZE_PARTS := fip:=$(STAGING_DIR_IMAGE)/u-boot.fip

Also add support for rootfs-in-uImage.FIT setups which don't require a
rootfs partition and make the (3rd) size parameter in UBINIZE_PARTS
optional (see example above without declared size).

Signed-off-by: Daniel Golle <daniel@makrotopia.org>