move ipxe image build to its own section too (team#40969)

Put `vmlinuz` and `initrdiso.img` on a public HTTPS endpoint. 

## iPXE booting

To boot a rescue system, you first need to build an iPXE image and

upload it to the BIOS. Then you can use the above image to boot a live

image.

At the time of writing (this might change!) the interface layout in

the iPXE environment is like this:

 * net0: management LAN

 * net1: public network

 * not detected: extra Intel gigabit network

Make sure the BIOS clock is up to date and that iPXE can ping and

resolve hosts on the Internet.

You might need to enable an interface before it works with:

    ifopen net1

You can check the open/closed status of the interfaces with:

    ifstat

## Modified iPXE image

You need to rebuild iPXE with `DOWNLOAD_PROTO_HTTPS` and UEFI support:

 #define IMAGE_ARCHIVE_CMD	/* Archive image management commands */

```

Then boot iPXE by going in the BIOS and ... TODO.

## iPXE booting

Then upload `ipxe.img` as a virtual floppy image and reboot. Press

<kbd>F11</kbd> to bring up the boot menu and select the `UEFI: ATEN

Virtual Floppy 3000` entry.

To boot a rescue system, you first need to build the above modified

iPXE image and upload it to the BIOS. Then you use the modified GRML

image to boot a live system.

At the time of writing (this might change!) the interface layout in

the iPXE environment is like this:

 * net0: management LAN

 * net1: public network

 * not detected: extra Intel gigabit network

Make sure the BIOS clock is up to date and that iPXE can ping and

resolve hosts on the Internet.

Then upload the `ipxe.img` built in the previous section as a virtual

floppy image and reboot. 

Press <kbd>F11</kbd> to bring up the boot menu and select the `UEFI:

ATEN Virtual Floppy 3000` entry.

Now, when `iPXE initialising devices...` is shown, frantically smash

the <kbd>ctrl-b</kbd> key combination, because the delay to drop to the

set net0/gateway 192.168.200.254

```

You might need to enable an interface before it works with:

    ifopen net1

You can check the open/closed status of the interfaces with:

    ifstat

Finally, boot the image:

```