Makefile: refactor include path settings

This commit merges commonly-used header include paths
to UBOOTINCLUDE and NOSTDINC_FLAGS variables, which are placed
at the top Makefile.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/Makefile b/Makefile
index 3a97483..9c39022 100644
--- a/Makefile
+++ b/Makefile
@@ -226,6 +226,15 @@
 CHECKFLAGS     := -D__linux__ -Dlinux -D__STDC__ -Dunix -D__unix__ \
 		  -Wbitwise -Wno-return-void -D__CHECK_ENDIAN__ $(CF)
 
+# Use UBOOTINCLUDE when you must reference the include/ directory.
+# Needed to be compatible with the O= option
+UBOOTINCLUDE    :=
+ifneq ($(OBJTREE),$(SRCTREE))
+UBOOTINCLUDE	+= -I$(OBJTREE)/include
+endif
+UBOOTINCLUDE	+= -I$(srctree)/include \
+		-I$(srctree)/arch/$(ARCH)/include
+
 KBUILD_CPPFLAGS := -D__KERNEL__
 
 KBUILD_CFLAGS   := -Wall -Wstrict-prototypes \
@@ -238,7 +247,7 @@
 export MAKE AWK
 export DTC CHECK CHECKFLAGS
 
-export KBUILD_CPPFLAGS
+export KBUILD_CPPFLAGS NOSTDINC_FLAGS UBOOTINCLUDE
 export KBUILD_CFLAGS KBUILD_AFLAGS
 
 KBUILD_CFLAGS += -Os #-fomit-frame-pointer
@@ -254,6 +263,9 @@
 # option to the assembler.
 KBUILD_AFLAGS	+= -g
 
+NOSTDINC_FLAGS += -nostdinc -isystem $(shell $(CC) -print-file-name=include)
+CHECKFLAGS     += $(NOSTDINC_FLAGS)
+
 # Report stack usage if supported
 KBUILD_CFLAGS += $(call cc-option,-fstack-usage)