This patch adds the following helper functions:
* delayed_postinsts - outputs the list of delayed postinstalls;
* save_postinsts - this will save the delayed postinstalls for ipk/deb
in /etc/(ipk|deb)_postinsts;
* rootfs_remove_packages - removes packages from an image;
Additionaly, this patch will remove a piece of code in
rootfs_ipk_do_rootfs which will be moved to image.bbclass and used for
all backends;
