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/spl/Makefile b/spl/Makefile
index 5339338..f273015 100644
--- a/spl/Makefile
+++ b/spl/Makefile
@@ -14,6 +14,8 @@
 # Based on top-level Makefile.
 #
 
+include $(srctree)/scripts/Kbuild.include
+
 CONFIG_SPL_BUILD := y
 export CONFIG_SPL_BUILD
 
@@ -127,8 +129,6 @@
 $(error could not find linker script)
 endif
 
-build := -f $(TOPDIR)/scripts/Makefile.build -C
-
 # Special flags for CPP when processing the linker script.
 # Pass the version down so we can handle backwards compatibility
 # on the fly.