sysvinit-inittab: ensure unique label for SERIAL_CONSOLES entries

sysvinit-inittab: ensure unique label for SERIAL_CONSOLES entries

The label field in /etc/inittab entries needs to be unique, and the
numeric label being used for the SERIAL_CONSOLES getty entries was
clashing with the entries added for standard ttyX entries added via
SYSVINIT_ENABLED_GETTYS. Use the part after "tty" in the device name
(which is what the comment further down explicitly says should be done)
as the label rather than a simple incrementing number.

Fixes [YOCTO #4374].

(From OE-Core rev: 28d3202befcec72554885f8ea9cb7985523b89f5)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>