bsp: Remove uboot-envtools for 4/32 devices

In OpenWrt 18.06.2, tiny devices have increased in size so that
another packages removal is requires.

Fixes #122.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Reviewed-by: Tim Niemeyer <tim@tn-x.org>
Reviewed-by: Christian Dresel <fff@chrisi01.de>