less: build after busybox

Build less after busybox if it's enabled, since we prefer the full-blown
version over the light one if it's enabled (and busybox would overwrite
the link when built afterwards).

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>