kbuild: remove 'addtree' and 'flags' magic for header search paths

The 'addtree' and 'flags' in scripts/Kbuild.include are so compilecated
and ugly.

As I mentioned in [1], Kbuild should stop automatic prefixing of header
search path options.

I fixed up (almost) all Makefiles in the kernel. Now 'addtree' and
'flags' have been removed.

Kbuild still caters to add $(srctree)/$(src) and $(objtree)/$(obj)
to the header search path for O= building, but never touches extra
compiler options from ccflags-y etc.

[1]: https://patchwork.kernel.org/patch/9632347/

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/scripts/Makefile.host b/scripts/Makefile.host
index 73b8041..b6a54bd 100644
--- a/scripts/Makefile.host
+++ b/scripts/Makefile.host
@@ -67,18 +67,16 @@
 _hostcxx_flags = $(KBUILD_HOSTCXXFLAGS) $(HOST_EXTRACXXFLAGS) \
                  $(HOSTCXXFLAGS_$(basetarget).o)
 
-__hostc_flags	= $(_hostc_flags)
-__hostcxx_flags	= $(_hostcxx_flags)
-
+# $(objtree)/$(obj) for including generated headers from checkin source files
 ifeq ($(KBUILD_EXTMOD),)
 ifneq ($(srctree),.)
-__hostc_flags	= -I$(obj) $(call flags,_hostc_flags)
-__hostcxx_flags	= -I$(obj) $(call flags,_hostcxx_flags)
+_hostc_flags   += -I $(objtree)/$(obj)
+_hostcxx_flags += -I $(objtree)/$(obj)
 endif
 endif
 
-hostc_flags    = -Wp,-MD,$(depfile) $(__hostc_flags)
-hostcxx_flags  = -Wp,-MD,$(depfile) $(__hostcxx_flags)
+hostc_flags    = -Wp,-MD,$(depfile) $(_hostc_flags)
+hostcxx_flags  = -Wp,-MD,$(depfile) $(_hostcxx_flags)
 
 #####
 # Compile programs on the host