rm_work: Speed up rootfs/populate_sdk removal

rm_work: Speed up rootfs/populate_sdk removal

Commands like bitbake X -c rootfs or bitbake X -c populate_sdk do not
trigger rm_work to clean up the directories afterwards since it
traditionally hooks onto do_build. This change means those two tasks now
clean up after themselves. We use the cleandirs function attribute to
handle this.

[YOCTO #6413]

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>