kbuild: restore arch/{ppc/xtensa}/boot cflags

Commit 9a39e273 removed the boot directory
addition to CFLAGS that was being used by the subdirectory builds.  For the
other files, that patch set EXTRA_CFLAGS, but Makefile.build explicitly
sets that to empty as it is explicitly for a single directory only.
Append to KBUILD_CFLAGS instead.
Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

arch/ppc/boot/Makefile

@@ -13,6 +13,8 @@
 # modified by Cort (cort@cs.nmt.edu)
 #
 
+# KBUILD_CFLAGS used when building rest of boot (takes effect recursively)
+KBUILD_CFLAGS 	+= -fno-builtin -D__BOOTER__ -Iarch/$(ARCH)/boot/include
 HOSTCFLAGS	+= -Iarch/$(ARCH)/boot/include
 
 BOOT_TARGETS	= zImage zImage.initrd znetboot znetboot.initrd

arch/xtensa/boot/Makefile

@@ -8,7 +8,8 @@
 #
 
 
-EXTRA_CFLAGS	+= -fno-builtin -Iarch/$(ARCH)/boot/include
+# KBUILD_CFLAGS used when building rest of boot (takes effect recursively)
+KBUILD_CFLAGS	+= -fno-builtin -Iarch/$(ARCH)/boot/include
 HOSTFLAGS	+= -Iarch/$(ARCH)/boot/include
 
 BIG_ENDIAN	:= $(shell echo -e __XTENSA_EB__ | $(CC) -E - | grep -v "\#")