init: properly parse kernel serial console options

According to https://www.kernel.org/doc/Documentation/admin-guide/serial-console.rst
the serial port options is specified as:
"... BBBBPNF, where BBBB is the speed, P is parity (n/o/e), N is number of bits,
and F is flow control ('r' for RTS)." Parity and Number of bits are ignored
since getty does not have any options for them.

I tested the paramater substitution using the following script/snippet:

```sh
/ # cat /etc/alpine-release; cat kernel2getty.sh; sh kernel2getty.sh
3.5.2
consoles="ttyS0 ttyS0,115200 ttyS0,115200n8 ttyS0,115200n8r ttyS0,115200r"
for console in $consoles; do
       line=-L
       term=vt100
       tty=${console%,*}
       speed=${console#*,}
       flow=${speed##*[^r]}
       speed=${speed%%[^0-9]*}
       echo "console=$console"
       echo "  speed=$speed"
       echo "   flow=$flow"
       echo "  getty=getty ${flow:+-h }$line ${speed:-15200} $tty $term"
       echo "----------"
done
console=ttyS0
  speed=
   flow=
  getty=getty -L 15200 ttyS0 vt100
----------
console=ttyS0,115200
  speed=115200
   flow=
  getty=getty -L 115200 ttyS0 vt100
----------
console=ttyS0,115200n8
  speed=115200
   flow=
  getty=getty -L 115200 ttyS0 vt100
----------
console=ttyS0,115200n8r
  speed=115200
   flow=r
  getty=getty -h -L 115200 ttyS0 vt100
----------
console=ttyS0,115200r
  speed=115200
   flow=r
  getty=getty -h -L 115200 ttyS0 vt100
----------
```

closes #7037

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