board/raspberrypi/post-build.sh: only tweak /etc/inittab if available

As pointed out in bug #9161, we don't always have an inittab file (if
systemd or no init is used), so the post build script should only try to
tweak it if present.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>