Makefile: pass include directives to dtc via CPPFLAGS, not CFLAGS

Makefile: pass include directives to dtc via CPPFLAGS, not CFLAGS

The dtc submodule's makefile expects -I include directives to be
in CPPFLAGS, not CFLAGS. Getting this wrong meant that the include
directive would not be applied when generating the dependency .d
files. This in turn meant that we couldn't build libfdt when
doing an out of tree build with clang. (gcc's dependency output
is slightly different from clang in this situation so it happened
to work.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>