mkconfig: deny messed up ARCH definition

Refuse to setup a platform if the command line ARCH= is not the same
as the one required for the board. This prevents any user with
prehistoric aliases from messing up their builds.

Reported in thread:
http://old.nabble.com/-U-Boot--Build-breaks-on-some-OMAP3-configs-to26132721.html

Inputs from: Mike Frysinger and Wolfgang Denk:
http://lists.denx.de/pipermail/u-boot/2009-November/063642.html



Cc: Wolfgang Denk <wd@denx.de>
Cc: Mike Frysinger <vapier@gentoo.org>
Cc: Anand Gadiyar <gadiyar@ti.com>
Cc: Dirk Behme <dirk.behme@googlemail.com>
Signed-off-by: Nishanth Menon <nm@ti.com>