rootfs_deb: Update IMAGE_ROOTFS variable usage to be consistent

rootfs_deb: Update IMAGE_ROOTFS variable usage to be consistent

update IMAGE_ROOTFS variable to fix inconsistent use of IMAGE_ROOTFS variable
content in by apt-native after the changes introducd in
2343808f7386cbd4c7b43f1b24e65e173f5ddf15.  This commit changed IMAGE_ROOTFS
from "${TMPDIR}/rootfs" to "${TMPDIR}/rootfs/${PN}" and that caused
IMAGE_ROOTFS to expand to ${TMPDIR}/rootfs/apt-native in apt.conf.sample,
however rootfs_deb.class uses apt.conf.sample to generate apt-rootfs.conf and
here here IMAGE_ROOTFS is different from that in apt-native package.

Acked-by: Tom Rini <trini@embeddedaley.com>