boot/uboot: remove deprecated network settings option

It's been deprecated since the 2014.05 release, so a year has passed.
Add legacy info as well to raise a warning when it's used.

[Peter: Remove entire config.h fixup handling]
Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>