kconfig: adapt update-po-config to new UML layout

Commit 5c48b108 ("um: take arch/um/sys-x86 to arch/x86/um") broke the
make target update-po-config, as its symlink trick (again) fails.
(Previous breakage was fixed with commit bdc69ca4 ("kconfig: change
update-po-config to reflect new layout of arch/um").)

The new UML layout allows to drop the symlick trick entirely. And if,
one day, another architecture supports UML too, that should now work
without again breaking this make target.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Michal Marek <mmarek@suse.cz>