kbuild: Use Kbuild.include

This commit adjusts some files to use Kbuild.include.

 - Use cc-option defined in Kbuild.include
    (Delete cc-option in config.mk)
 - Use cc-version defined in
    (Delete cc-version in config.mk)
 - Move binutils-version and dtc-version to Kbuild.include
     by analogy to cc-version

This commit also adds srctree (same as SRCTREE)
to use Kbuild scripts.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/Makefile b/Makefile
index 0c1eef3..e4045bc 100644
--- a/Makefile
+++ b/Makefile
@@ -102,9 +102,10 @@
 SPLTREE		:= $(OBJTREE)/spl
 TPLTREE		:= $(OBJTREE)/tpl
 SRCTREE		:= $(CURDIR)
+srctree		:= $(SRCTREE)
 TOPDIR		:= $(SRCTREE)
 LNDIR		:= $(OBJTREE)
-export	TOPDIR SRCTREE OBJTREE SPLTREE TPLTREE
+export	TOPDIR SRCTREE srctree OBJTREE SPLTREE TPLTREE
 
 MKCONFIG	:= $(SRCTREE)/mkconfig
 export MKCONFIG
@@ -126,8 +127,6 @@
 
 #########################################################################
 
-build := -f $(TOPDIR)/scripts/Makefile.build -C
-
 # The "tools" are needed early, so put this first
 # Don't include stuff already done in $(LIBS)
 # The "examples" conditionally depend on U-Boot (say, when USE_PRIVATE_LIBGCC
@@ -198,6 +197,10 @@
 HOSTLDFLAGS += $(call os_x_before, 10, 5, "-multiply_defined suppress")
 endif
 
+# We need some generic definitions (do not try to remake the file).
+$(srctree)/scripts/Kbuild.include: ;
+include $(srctree)/scripts/Kbuild.include
+
 # Make variables (CC, etc...)
 
 AS		= $(CROSS_COMPILE)as