um: Fix for relative objtree when generating x86 headers

In an O= build, rely on the generated Makefile to call the main Makefile
properly. When building in the source tree, we do not need to specify
the -C and O= either. This fixes the problem when $(objtree) is a
relative path and the -C changes the directory.

Reported-by: Richard Weinberger <richard.weinberger@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>