mkconfig: change CONFIG_MK_ prefix into plain CONFIG_

When planning for more generalization and Makefile cleanup it became
obvious that the introduction of a separate CONFIG_MK_ name space for
config options that were set through scripting in the Makefile was
not a good idea.

Originally the idea was to provide a script-free approach to supply
configuration options - there was no real need for a separate name
space. But when we now convert the existing Makefile entries to make
use of this approach, it would mean that we have to touch a large
number of board config files and add #ifdef / #define sequences to
"convert" from the CONFIG_MK_ to the CONFIG_ name space.

It seems much cleaner to get rid of this somewhat arbitrary _MK
string now for the few boards that actually use it.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Acked-by: Mike Frysinger <vapier@gentoo.org>