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>