kbuild: do not require sub-make for separate output tree builds

As explained in commit 3204a7fb ("kbuild: prefix $(srctree)/ to some
included Makefiles"), I want to stop using --include-dir some day.

I already fixed up the top Makefile, but some arch Makefiles (mips, um,
x86) still include check-in Makefiles without $(srctree)/.

Fix them up so 'need-sub-make := 1' can go away for this case.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Makefile

@@ -191,10 +191,9 @@ endif
 ifneq ($(abs_srctree),$(abs_objtree))
 # Look for make include files relative to root of kernel src
 #
-# This does not become effective immediately because MAKEFLAGS is re-parsed
-# once after the Makefile is read. We need to invoke sub-make.
+# --included-dir is added for backward compatibility, but you should not rely on
+# it. Please add $(srctree)/ prefix to include Makefiles in the source tree.
 MAKEFLAGS += --include-dir=$(abs_srctree)
-need-sub-make := 1
 endif
 
 ifneq ($(filter 3.%,$(MAKE_VERSION)),)

arch/mips/Makefile

@@ -254,7 +254,7 @@ endif
 #
 # Board-dependent options and extra files
 #
-include arch/mips/Kbuild.platforms
+include $(srctree)/arch/mips/Kbuild.platforms
 
 ifdef CONFIG_PHYSICAL_START
 load-y					= $(CONFIG_PHYSICAL_START)

arch/um/Makefile

@@ -41,8 +41,8 @@ endif
 
 HOST_DIR := arch/$(HEADER_ARCH)
 
-include $(ARCH_DIR)/Makefile-skas
-include $(HOST_DIR)/Makefile.um
+include $(srctree)/$(ARCH_DIR)/Makefile-skas
+include $(srctree)/$(HOST_DIR)/Makefile.um
 
 core-y += $(HOST_DIR)/um/
 
@@ -76,7 +76,7 @@ USER_CFLAGS = $(patsubst $(KERNEL_DEFINES),,$(patsubst -I%,,$(KBUILD_CFLAGS))) \
 		-idirafter $(objtree)/include -D__KERNEL__ -D__UM_HOST__
 
 #This will adjust *FLAGS accordingly to the platform.
-include $(ARCH_DIR)/Makefile-os-$(OS)
+include $(srctree)/$(ARCH_DIR)/Makefile-os-$(OS)
 
 KBUILD_CPPFLAGS += -I$(srctree)/$(HOST_DIR)/include \
 		   -I$(srctree)/$(HOST_DIR)/include/uapi \

arch/x86/Makefile

@@ -75,7 +75,7 @@ ifeq ($(CONFIG_X86_32),y)
         KBUILD_CFLAGS += $(call cc-option,$(cc_stack_align4))
 
         # CPU-specific tuning. Anything which can be shared with UML should go here.
-        include arch/x86/Makefile_32.cpu
+        include $(srctree)/arch/x86/Makefile_32.cpu
         KBUILD_CFLAGS += $(cflags-y)
 
         # temporary until string.h is fixed