also removed function board_clean from bsp files and board_postbuild from buildscript

Signed-off-by: Jan Kraus <[email protected]>

suggested-by: Alexander Wunschik <[email protected]>
Co-Authored-by: Alexander Wunschik <[email protected]>
Co-Authored-by: Tobias Klaus <[email protected]>

Reviewed-by: Tim Niemeyer <[email protected]>
Reviewed-by: Tobias Klaus <[email protected]>