kernel: Stop using update-alternatives

kernel: Stop using update-alternatives

The update-alternatives where using relative links so not being really
in use since December 2016 (see OE-Core:c7bc46b9 "kernel: Fix
symlinks") so instead we now generate the relative symlinks during the
do_install task and drop the update-alternatives use at all.

Acked-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Ross Burton <ross.burton@intel.com>
(cherry picked from commit 10a1b293191268e6792ac8e27bd6427f1974c7ce)
Signed-off-by: Andreas Oberritter <obi@opendreambox.org>