rootfs_*.bbclass: add some helper functions

rootfs_*.bbclass: add some helper functions

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;

[YOCTO #4484]

(From OE-Core rev: 928df79cd3964f775c4c6e4283ef84b8882f9328)

Signed-off-by: Laurentiu Palcu <laurentiu.palcu@intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>