busybox-inittab: Implement SYSVINIT_ENABLED_GETTYS and USE_VT

This ensures that busybox init can generate matching tty's in inittab as
system defines, secondly resepcts USE_VT for creating virtual ttys

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>