multilib.bbclass: Filter ROOTFS_RO_UNNEEDED to fix uninstallation

multilib.bbclass: Filter ROOTFS_RO_UNNEEDED to fix uninstallation

When building an entire multilib image (e.g. lib32-core-image-minimal)
we need to ensure that the unneeded packages in ROOTFS_RO_UNNEEDED get
the multilib prefix applied before they are compared against the list of
installed packages inside Rootfs._uninstall_unneeded() to decide whether
they need to be installed.

Before this change with local.conf containing:

 MACHINE = qemuarm64
 require conf/multilib.conf
 MULTILIBS = "multilib:lib32"
 DEFAULTTUNE:virtclass-multilib-lib32 = "armv7at-neon"
 IMAGE_FEATURES = "read-only-rootfs"

running:
 bitbake lib32-core-image-minimal
 tar tavf tmp/deploy/images/qemuarm64/lib32-core-image-minimal-qemuarm64.rootfs.tar.zst|grep postinst

shows three files:
 -rwxr-xr-x 0/0              25 2018-03-09 12:34 ./etc/init.d/run-postinsts
 lrwxrwxrwx 0/0               0 2018-03-09 12:34 ./etc/rcS.d/S99run-postinsts -> ../init.d/run-postinsts
 -rwxr-xr-x 0/0            2153 2018-03-09 12:34 ./usr/sbin/run-postinsts

whereas with this change there are no matching files.

Signed-off-by: Mike Crowe <mac@mcrowe.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>