package/rauc: disable systemd for host build

Since there is not necessary to have support of systemd within the host
variant let's disable it unconditionally to solve the following errors:

/usr/bin/install -c -m 644 data/rauc.service '/usr/lib/systemd/system'
/usr/bin/install: cannot create regular file '/usr/lib/systemd/system/rauc.service': Permission denied
/usr/bin/install -c -m 644 data/de.pengutronix.rauc.conf 'no'
make[4]: *** [Makefile:1700: install-nodist_systemdunitDATA] Error 1
make[4]: *** Waiting for unfinished jobs....

Signed-off-by: Bartosz Bilas <b.bilas@grinn-global.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>