console redirection parameters cannot be easily modified

The charm sets the following GRUB_CMDLINE by default:

GRUB_CMDLINE_LINUX_DEFAULT="$GRUB_CMDLINE_LINUX_DEFAULT console=tty0 console=ttyS0,115200 console=ttyS1,115200"

If the motherboard is configured to redirect the output onto a different COM port, or if the operator prefers to have foreground messages over serial instead of VGA/iKVM, they have to override GRUB_CMDLINE_LINUX_DEFAULT via grub-config-flags.

This could be made more user-friendly by providing new config options. For example:

  console-serial-port:
    default: ""
    type: string
    description: |
        Console output can be redirected to a serial port.
        Specify which one here, using either the ttyS<n>
        device name or a COM port.
        COM ports will be converted to tty device names
        as follows: COM1 = ttyS0, COM2 = ttyS1, etc.
  foreground-console:
    default: "vga"
    type: string
    description: |
        Output foreground messages on either vga or serial.
        NOTE: "vga" includes iKVM virtual consoles found on
        the BMC web UIs
        Valid options: [vga|serial]