image.bbclass: fix rootfs generation without package management tools
When building without package management tools the
ROOTFS_BOOTSTRAP_INSTALL packages need to be available to provide the
bare minimal for rootfs unconfigure postinsts to be on first boot.
Those packages where being include in core-image.bbclass' based images
however every image needs those available for proper rootfs generation.
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
