uml: use DEFCONFIG_LIST to avoid reading host's config

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.

Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Acked-by: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>