nixos/make-disk-image: support partition layout `"legacy+boot"`

This patch adds a new partition layout type `"legacy+boot` which will
produce MBR images with a separate FAT32 boot partition. This is
necessary for adding a NixOS test for Limine on BIOS systems, as Limine
can't read ext4 partitions.

nixos/lib/make-disk-image.nix

···
 
   This partition layout is unsuitable for UEFI.
 
+  #### `legacy+boot`
+
+  The image is partitioned using MBR and:
+  - creates a FAT32 BOOT partition from 1MiB to specified `bootSize` parameter (256MiB by default), set it bootable ;
+  - creates a primary ext4 partition starting after the boot partition and extending to the full disk image
+
+  This partition layout is unsuitable for UEFI.
+
   #### `legacy+gpt`
 
   This partition table type uses GPT and:
···
   additionalSpace ? "512M",
 
   # size of the boot partition, is only used if partitionTableType is
-  # either "efi" or "hybrid"
+  # either "efi", "hybrid", or "legacy+boot"
   # This will be undersized slightly, as this is actually the offset of
   # the end of the partition. Generally it will be 1MiB smaller.
   bootSize ? "256M",
···
 assert (
   lib.assertOneOf "partitionTableType" partitionTableType [
     "legacy"
+    "legacy+boot"
     "legacy+gpt"
     "efi"
     "efixbootldr"
···
     {
       # switch-case
       legacy = "1";
+      "legacy+boot" = "2";
       "legacy+gpt" = "2";
       efi = "2";
       efixbootldr = "3";
···
           mkpart primary ext4 1MiB 100% \
           print
       '';
+      "legacy+boot" = ''
+        parted --script $diskImage -- \
+          mklabel msdos \
+          mkpart primary fat32 1MiB $bootSizeMiB \
+          set 1 boot on \
+          mkpart primary ext4 $bootSizeMiB 100% \
+          print
+      '';
       "legacy+gpt" = ''
         parted --script $diskImage -- \
           mklabel gpt \
···
                 # Add the 1MiB aligned reserved space (includes MBR)
                 reservedSpace=$(( mebibyte ))
               ''
+            else if partitionTableType == "legacy+boot" then
+              ''
+                # The explanation from the above "efi" case applies here too,
+                # but gptSpace is not needed without a GPT.
+                reservedSpace=$(( bootSize ))
+              ''
             else
               ''
                 reservedSpace=0
···
           mount /dev/vda2 /mnt/boot
 
           ${lib.optionalString touchEFIVars "mount -t efivarfs efivarfs /sys/firmware/efi/efivars"}
+        ''}
+        ${lib.optionalString (partitionTableType == "legacy+boot") ''
+          mkdir -p /mnt/boot
+          mkfs.vfat -n BOOT /dev/vda1
+          mount /dev/vda1 /mnt/boot
         ''}
 
         # Install a configuration.nix