runqueue: Ensure we do run 'masked' setscene tasks if specified as targets

If you specify multiple targets on bitbake's commandline and some of them are
setscene tasks which are "masked" by other tasks they may not get run.

For example <image>:do_rootfs <kernel>:do_populate_sysroot

the rootfs tasks "masks" the populate_sysroot task so bitbake would currently
decide not to run it. In this case, we do really want it to be run.

The fix is not to skip anything which has been given as an explict target.

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