kbuild: add arch/$ARCH/include to search path

This patch conclude the support for

arch/$ARCH/include

Note: The individual architectures will most likely require
a few minor patches to support locating header files in
arch/$ARCH/include

Testing shows that it worked out-of-the-box for sparc.
x86 required a few trivial changes in the arch
specific Makefile and a few include paths had to be adjusted.

Signed-off-by: Sam Ravnborg <[e-mail]>

Makefile

···
 # Needed to be compatible with the O= option
 LINUXINCLUDE    := -Iinclude \
                    $(if $(KBUILD_SRC),-Iinclude2 -I$(srctree)/include) \
-		   -include include/linux/autoconf.h
+                   -I$(srctree)/arch/$(hdr-arch)/include               \
+                   -include include/linux/autoconf.h
 
 KBUILD_CPPFLAGS := -D__KERNEL__ $(LINUXINCLUDE)