OpenWrt: Save space

- make USB optional
- remove uboot-env tools for mr3020

Signed-off-by: Tim Niemeyer <tim@tn-x.org>
[fabian@blaese.de: Don't strip libs]
Signed-off-by: Fabian Bläse <fabian@blaese.de>
Reviewed-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>