sysvinit-inittab/start_getty: Check /sys for the tty device existence

The hvc tty driver doesn't populate a file like /proc/tty/driver/serial,
so the current implementation of start_getty doesn't work for the hvc
console. By checking the /sys/class/tty/ for the tty device existence,
it should support more console types and also make the codes more simple.

Signed-off-by: Kevin Hao <kexin.hao@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>