x86, um: get rid of arch/um/Kconfig.arch

Teach scripts/kconfig/Makefile and top-level Makefile that arch/*/Makefile
is allowed to say Kconfig := <whatever I want instead of arch/blah/Kconfig>.
Rewrite arch/um/Kconfig and arch/um/Kconfig.<subarch> so that the latter
would be top-level one (and include the pieces of the former).

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>