image-live.bbclass: order do_bootimg after do_rootfs

do_bootimg expects IMGDEPLOYDIR to exist, since it stores its artifacts
there. Therefore, do_bootimg should run after do_rootfs because
IMGDEPLOYDIR is created before do_rootfs runs since IMGDEPLOYDIR is
contained in do_rootfs' [cleandirs] varflag.

When do_bootimg depends on ${PN}:do_image_${LIVE_ROOTFS_TYPE},
do_bootimg is correctly ordered after do_rootfs because
do_image_${FSTYPE} tasks are added after do_image and do_image itself is
added after do_rootfs.

However, when do_bootimg doesn't depend on
${PN}:do_image_${LIVE_ROOTFS_TYPE}
(introduced by: 96f47c39), do_bootimg
can run before do_rootfs, thus before IMGDEPLOYDIR is created. To
avoid this situation, do_bootimg is now explicitly ordered after
do_rootfs.

Signed-off-by: Guillaume Champagne <champagne.guillaume.c@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 73c21db8)
Signed-off-by: Steve Sakoman <steve@sakoman.com>