systemd: fix runtime dependency

systemd: fix runtime dependency

Previously, if we set 'NO_RECOMMENDATIONS' to "1", we cannot reach login
in systemd systems. The problem is that several runtime dependencies are
coded as 'recommended' in our project.

This patch makes the following changes.
1. Add util-linux-agetty as a runtime dependency.
   Otherwise, getty services in systemd cannot start correctly.
2. Add systemd-serialgetty as a runtime dependency.
   Otherwise, serialgetty services cannot start correctly in systemd and
   we would have problem when using `runqemu qemux86 nographic'.
3. Remove 'systemd' from RDEPENDS of systemd-serialgetty.
   systemd-serialgetty justs ships systemd unit files, even if the init
   manager is 'sysvinit', these files do no harm.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>