syslinux: change serial bit rate to 115200

Prior to this commit an installation over serial via syslinux would
involve:
1. setting bitrate to BIOS's bitrate (typically 115200)
2. setting bitrate to syslinux's bitrate (38400)
3. setting bitrate to stty's bitrate (115200)

By changing syslinux's bitrate to 115200, an installation over serial
is a smoother experience, and consistent with the GRUB2 installation
which is also 115200 bps.

    [root@nixos:~]# stty
    speed 115200 baud; line = 0;
    -brkint ixoff iutf8
    -iexten

In a future commit I will add default serial terminals to the syslinux
kernel lines.