iso-image: add 'serial console' boot entry

Prior to this commit, installation over serial console would requiring
manually having to modify the kernel modeline, as described in
https://github.com/NixOS/nixpkgs/issues/58198 .

This is unnecessarily fiddly, so this commit adds a syslinux boot
entry that has serial enabled.

GRUB already has a serial console entry:
2c07a0800a/nixos/modules/installer/cd-dvd/iso-image.nix (L311-L317)

Why 115200 bps? This is already used in other places, e.g. https://github.com/NixOS/nixpkgs/pull/58196

I tested this change by building the image, booting the image, and
observing the boot process over serial:

    $ cd nixos/
    $ nix-build -A config.system.build.isoImage -I nixos-config=modules/installer/cd-dvd/installation-cd-minimal.nix default.nix
    $ sudo cp /nix/store/arcl702c3z8xlndlvnfplq9yhixjvs9k-nixos-20.09pre-git-x86_64-linux.iso/iso/nixos-20.09pre-git-x86_64-linux.iso /dev/sdb

    $ picocom -b 115200 /dev/ttyUSB0

nixos/modules/installer/cd-dvd/iso-image.nix

@@ -143,6 +143,13 @@ let
     LINUX /boot/${config.system.boot.loader.kernelFile}
     APPEND init=${config.system.build.toplevel}/init ${toString config.boot.kernelParams} loglevel=7
     INITRD /boot/${config.system.boot.loader.initrdFile}
+
+    # A variant to boot with a serial console enabled
+    LABEL boot-serial
+    MENU LABEL NixOS ${config.system.nixos.label}${config.isoImage.appendToMenuLabel} (serial console=ttyS0,115200n8)
+    LINUX /boot/${config.system.boot.loader.kernelFile}
+    APPEND init=${config.system.build.toplevel}/init ${toString config.boot.kernelParams} console=ttyS0,115200n8
+    INITRD /boot/${config.system.boot.loader.initrdFile}
   '';
 
   isolinuxMemtest86Entry = ''