kbuild: use relative path more to include Makefile

Prior to this commit, it was impossible to use relative path to
include Makefiles from the top level Makefile because the option
"--include-dir=$(srctree)" becomes effective when Make enters into
sub Makefiles.

To use relative path in any places, this commit moves the option
above the "sub-make" target.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>

arch/um/Makefile

@@ -43,8 +43,8 @@ endif
 
 HOST_DIR := arch/$(HEADER_ARCH)
 
-include $(srctree)/$(ARCH_DIR)/Makefile-skas
-include $(srctree)/$(HOST_DIR)/Makefile.um
+include $(ARCH_DIR)/Makefile-skas
+include $(HOST_DIR)/Makefile.um
 
 core-y += $(HOST_DIR)/um/
 
@@ -73,7 +73,7 @@ USER_CFLAGS = $(patsubst $(KERNEL_DEFINES),,$(patsubst -D__KERNEL__,,\
 	$(filter -I%,$(CFLAGS)) -D_FILE_OFFSET_BITS=64 -idirafter include
 
 #This will adjust *FLAGS accordingly to the platform.
-include $(srctree)/$(ARCH_DIR)/Makefile-os-$(OS)
+include $(ARCH_DIR)/Makefile-os-$(OS)
 
 KBUILD_CPPFLAGS += -I$(srctree)/$(HOST_DIR)/include \
 		   -I$(srctree)/$(HOST_DIR)/include/uapi \