xen: use PACKAGECONFIG for SDL and a bit of dependency cleanup

This patch moves the optional SDL dependency to a PACKAGECONFIG
entry. It also cleans up dependencies on both the virtual libsdl
and libsdl. The dependency on libgl is also removed. libGL support
should be enabled with the 'opengl' distro feature instead. glib-2.0
dependency is now explicit as well (previously pulled in by libsdl).
Add getty to inittab for login on Xen serial console.

It's not pretty but the mechanisms in sysvinit-inittab assume the
console device name is tty*. The Xen console device doesn't meet
this criteria.