loader: Allow this to compile w/o MD_IMAGE_SIZE defined

[imp]

Summary

Slight tweak to allow md.c to be compiled when MD_IMAGE_SIZE isn't
defined.

Sponsored by: Netflix

[olivier]

Working great!

First a simple shell script (requiere qemu and ipxe firmwares):

#!/bin/sh
# Boot a stock FreeBSD memstick over the network using the D59415..D59421
# loader
#
# The patched loader.efi speaks iPXE's download protocol itself: iPXE only
# chainloads it, and the loader then fetches the memstick straight into RAM,
# registers it, and boots.  Nothing is typed at the loader prompt.
#
# Both files are fetched over HTTPS from their public homes; the memstick is
# the stock published snapshot, unmodified.

# Requirements:
# - qemu
# - ipxe

set -eu

loader=https://people.freebsd.org/~olivier/iPXE/loader.D59415.efi
snap=FreeBSD-16.0-CURRENT-amd64-20260824-74019bc3ea91-288532
image=https://download.freebsd.org/ftp/snapshots/ISO-IMAGES/16.0/$snap-mini-memstick.img.xz
arch=x86_64
mem=8G                  # the image lives in RAM; 726MB plus headroom
workdir=$(mktemp -d /tmp/d59415.XXXXXX)
trap 'rm -rf "$workdir"' EXIT INT TERM

# iPXE hardcodes "file:autoexec.ipxe", so this must keep that exact name.
# memdisk= is for a disk image (a memstick); an ISO would use memcd=.
cat > "$workdir/autoexec.ipxe" <<EOF
#!ipxe
dhcp
echo === chainloading patched loader.efi ===
chain $loader memdisk=$image
echo Boot failed, press a key
prompt
EOF

cp /usr/local/share/ipxe/ipxe.efi-${arch} "$workdir/ipxe.efi"
for f in /usr/local/share/qemu/edk2-${arch}-code.fd \
         /usr/local/share/edk2-qemu/QEMU_UEFI_CODE-${arch}.fd
do [ -f "$f" ] && { efi=$f; break; }; done
for v in /usr/local/share/qemu/edk2-i386-vars.fd \
         /usr/local/share/edk2-qemu/QEMU_UEFI_VARS-${arch}.fd
do [ -f "$v" ] && { varstpl=$v; break; }; done
cp "$varstpl" "$workdir/vars.fd"; chmod u+w "$workdir/vars.fd"

# -device virtio-rng-pci is REQUIRED: the TLS-enabled EDK2 network stack
# blocks on entropy and never sends DHCP without it.  SLIRP gives the guest
# working outbound NAT, which is what reaches people.freebsd.org.
exec qemu-system-${arch} -m $mem -boot n \
  -drive if=pflash,unit=0,readonly=on,format=raw,file="$efi" \
  -drive if=pflash,unit=1,format=raw,file="$workdir/vars.fd" \
  -netdev user,id=net0,tftp="$workdir",bootfile=ipxe.efi \
  -device virtio-net-pci,netdev=net0 \
  -device virtio-rng-pci \
  -display none -serial mon:stdio

Then run it:

$ ./boot.sh

>>Start PXE over IPv4.
  Station IP address is 10.0.2.15

  Server IP address is 10.0.2.2
  NBP filename is ipxe.efi
  NBP filesize is 1170432 Bytes
 Downloading NBP file...

  NBP file downloaded successfully.
BdsDxe: loading Boot0001 "UEFI PXEv4 (MAC:525400123456)" from PciRoot(0x0)/Pci(0x3,0x0)/MAC(525400123456,0x1)/IP
v4(0.0.0.0,0x0,DHCP,0.0.0.0,0.0.0.0,0.0.0.0)
BdsDxe: starting Boot0001 "UEFI PXEv4 (MAC:525400123456)" from PciRoot(0x0)/Pci(0x3,0x0)/MAC(525400123456,0x1)/I
Pv4(0.0.0.0,0x0,DHCP,0.0.0.0,0.0.0.0,0.0.0.0)
iiPPXXEE  iinniittiiaalliissiinngg  ddeevviicceess......

aauuttooeexxeecc..iippxxee......  ookk
Consoles: EFI console
Downloading https://download.freebsd.org/ftp/snapshots/ISO-IMAGES/16.0/FreeBSD-16.0-CURRENT-amd64-20260824-74019bc3ea91-288532-mini-memstick.img.xz as a disk
We know we will download 126731440 bytes
Staritng the download
xz
126MB / 126MB (99%)
Downloaded 126731440 bytes, actual size 726782464 -- registering ramdisk
Installed RAM disk as VirtualDisk(0x6C2CF000,0x977EC1FF,0)
    Reading loader env vars from /efi/freebsd/loader.env
FreeBSD/amd64 EFI loader, Revision 3.0
(Fri Sep  4 21:52:44 CEST 2026 olivier@bigone)
Command line arguments: loader.D59415.efi memdisk=https://download.freebsd.org/ftp/snapshots/ISO-IMAGES/16.0/
FreeBSD-16.0-CURRENT-amd64-20260824-74019bc3ea91-288532-mini-memstick.img.xz
   Image base: 0xbd962000
   EFI version: 2.70
   EFI Firmware: EDK II (rev 1.00)
   Console: efi (0x20001000)
   Load Path: loader.D59415.efi
   Load Device: PciRoot(0x0)/Pci(0x3,0x0)/MAC(525400123456,0x1)/Uri(https://people.freebsd.org/~olivier/iPXE/loa
der.D59415.efi)
    Ignoring UEFI boot manager
    Trying uefi_rootdev VirtualDisk(0x6C2CF000,0x977EC1FF,0)
Trying: VirtualDisk(0x6C2CF000,0x977EC1FF,0)
Setting currdev to disk0:
Loading /boot/defaults/loader.conf
Loading /boot/defaults/loader.conf
Loading /boot/device.hints
Loading /boot/loader.conf
Loading /boot/loader.conf.d/rtw88.conf
Loading /boot/loader.conf.local
-
-
\ _____              ____ ____  ____     ___           _        _ _
 |  ___| __ ___  ___| __ ) ___||  _ \   |_ _|_ __  ___| |_ __ _| | | ___ _ __
 | |_ | '__/ _ \/ _ \  _ \___ \| | | |   | || '_ \/ __| __/ _` | | |/ _ \ '__|
 |  _|| | |  __/  __/ |_) |__) | |_| |   | || | | \__ \ || (_| | | |  __/ |
 |_|  |_|  \___|\___|____/____/|____/   |___|_| |_|___/\__\__,_|_|_|\___|_|

 ╔════════ Welcome to FreeBSD ══════════╗     ```                        `
 ║                                      ║    s` `.....---.......--.```   -/
 ║  1. Boot Installer [Enter]           ║    +o   .--`         /y:`      +.
 ║  2. Boot Single user                 ║     yo`:.            :o      `+-
 ║  3. Escape to loader prompt          ║      y/               -/`   -o/
 ║  4. Reboot                           ║     .-                  ::/sy+:.
 ║  5. Cons: Dual (Serial primary)      ║     /                     `--  /
 ║                                      ║    `:                          :`
 ║  Kernel:                             ║    `:                          :`
 ║  6. kernel (1 of 1)                  ║     /                          /
 ║                                      ║     .-                        -.
 ║  Options:                            ║      --                      -.
 ║  7. Boot Options                     ║       `:`                  `:`
 ║                                      ║         .--             `--.
 ╚══════════════════════════════════════╝            .---.....----.|

Loading kernel...
/boot/kernel/kernel text=0x19cb20 text=0xf1930c text=0x4cffd7 data=0x180+0xe80 data=0x1b2f68+0x44d098 0x8+0x1b70
08+0x8+0x1814af
(etc.)
Starting primary installer on ttyu0

Welcome to FreeBSD!

Please choose the appropriate terminal type for your system.
Common console types are:
   ansi     Standard ANSI terminal
   vt100    VT100 or compatible terminal
   xterm    xterm terminal emulator (or compatible)

Console type [vt100]: