systemd: add SYSTEMD_CPPFLAGS for local kernel headers

systemd uses cpp to generate tables for keymappings. Without setting
CPPFLAGS the toolchain headers will be used for generation but
local kernel headers for compilation. This can break compile.

Since CPPFLAGS are implicitly used when calling compiler directly,
we can drop the CFLAGS settings for local kernel headers.

tested with kernel 3.9

Signed-off-by: Markus Niebel <Markus.Niebel@tq-group.com>
Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>