image-live.bbclass: LIVE_ROOTFS_TYPE support compression

image-live.bbclass: LIVE_ROOTFS_TYPE support compression

The task for fstypes with compression is the same as the task for the
uncompressed fstypes, e.g. when adding tar.xz to `IMAGE_FSTYPES`, it will
be included into the do_image_tar task and not creating a separate
do_image_tar.xz task.

This commit fixes `LIVE_ROOTFS_TYPE` with compressed fstypes by
depending on the actual task instead of the non-existent
do_image_<fstype>.<compression> task.

Fixes [YOCTO #15331]

Signed-off-by: Ludovic Jozeau <ludovic.jozeau@smile.fr>
Reviewed-by: Yoann Congal <yoann.congal@smile.fr>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>