image: Drop before do_build from do_rootfs and do_image

image: Drop before do_build from do_rootfs and do_image

do_image_complete has the do_build dependency, these other two tasks
don't need it. If this is present, it complicates the dependency tree
and convinces rm_work that the tasks need to rerun even if they
already did since the stamps are removed.

Drop the unneeded dependency to help resolve this issue.

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