image-live.bbclass: optional depends when ROOTFS empty

`ROOTFS` is optional. It can be empty if the live image doesn't require
a rootfs.  In such cases, the build doesn't depend on
`do_image_{LIVE_ROOTFS_TYPE}`.

Signed-off-by: Guillaume Champagne <champagne.guillaume.c@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>