buildroot

Mention the saveconfig and getconfig targets in make help

I often find myself forgetting what those targets were called, and I imagine
others may not even be aware of their existence.

Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>
Signed-off-by: Hans-Christian Egtvedt <hans-christian.egtvedt@atmel.com>