also removed function board_clean from bsp files and board_postbuild from buildscript

Signed-off-by: Jan Kraus <mayosemmel@gmail.com>

suggested-by: Alexander Wunschik <freifunk@wunschik.net>
Co-Authored-by: Alexander Wunschik <freifunk@wunschik.net>
Co-Authored-by: Tobias Klaus <tk+ff@meskal.net>

Reviewed-by: Tim Niemeyer <tim@tn-x.org>
Reviewed-by: Tobias Klaus <tk+ff@meskal.net>