[PATCH] uml: use DEFCONFIG_LIST to avoid reading host's config

This should make sure that, for UML, host's configuration files are not
considered, which avoids various pains to the user.  Our dependency are such
that the obtained Kconfig will be valid and will lead to successful
compilation - however they cannot prevent an user from disabling any boot
device, and if an option is not set in the read .config (say
/boot/config-XXX), with make menuconfig ARCH=um, it is not set.  This always
disables UBD and all console I/O channels, which leads to non-working UML
kernels, so this bothers users - especially now, since it will happen on
almost every machine (/boot/config-`uname -r` exists almost on every machine).
 It can be workarounded with make defconfig ARCH=um, but it is non-obvious and
can be avoided, so please _do_ merge this patch.

Given the existence of options, it could be interesting to implement
(additionally) "option required" - with it, Kconfig will refuse reading a
.config file (from wherever it comes) if the given option is not set.  With
this, one could mark with it the option characteristic of the given
architecture (it was an old proposal of Roman Zippel, when I pointed out our
problem):

config UML
	option required
	default y

However this should be further discussed:
*) for x86, it must support constructs like:

==arch/i386/Kconfig==
config 64BIT
	option required
	default n
where Kconfig must require that CONFIG_64BIT is disabled or not present in the
read .config.

*) do we want to do such checks only for the starting defconfig or also for
   .config? Which leads to:
*) I may want to port a x86_64 .config to x86 and viceversa, or even among more
   different archs. Should that be allowed, and in which measure (the user may
   force skipping the check for a .config or it is only given a warning by
   default)?

Cc: Roman Zippel <zippel@linux-m68k.org>
Cc: <kbuild-devel@lists.sourceforge.net>
Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Cc: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>