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>

arch/um/Makefile

@@ -111,8 +111,7 @@ endef
 KBUILD_KCONFIG := $(HOST_DIR)/um/Kconfig
 
 archheaders:
-	$(Q)$(MAKE) -C '$(srctree)' KBUILD_SRC= \
-		ARCH=$(HEADER_ARCH) O='$(objtree)' archheaders
+	$(Q)$(MAKE) KBUILD_SRC= ARCH=$(HEADER_ARCH) archheaders
 
 archprepare: include/generated/user_constants.h