busybox: use ttyrun to run getty only if the terminal exists

busybox: use ttyrun to run getty only if the terminal exists

Wrap calls to start_getty with ttyrun, so that getty isn't started if
the device doesn't exist.  As we know start_getty is only called when
the device exists we can remove the partial workaround for this problem
in that scripts too.

This neatly obsoletes SERIAL_CONSOLES_CHECK, whose sole purpose was to
check what terminals are present at boot and rewrite inittab.  Notably,
this meant that SERIAL_CONSOLES_CHECK made using a read-only rootfs
impossible.

(From OE-Core rev: 950ecaabc04836efc346be0ac7e0331e2378872b)

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>