nixos: iso-image: use syslinux bootloader for USB booting support

This changes the bootloader for iso generation from Grub to
syslinux. In addition this adds USB booting support, so that
"dd" can be used to burn the generated ISO to USB thumbdrives
instead of needing applications like UnetBootin.

nixos/lib/make-iso9660-image.sh

@@ -31,11 +31,20 @@ if test -n "$bootable"; then
         fi
     done
 
-    bootFlags="-b $bootImage -c .boot.cat -no-emul-boot -boot-load-size 4 -boot-info-table"
+    isoBootFlags="-eltorito-boot ${bootImage}
+                  -eltorito-catalog .boot.cat
+                  -no-emul-boot -boot-load-size 4 -boot-info-table"
+fi
+
+if test -n "$usbBootable"; then
+    usbBootFlags="-isohybrid-mbr ${isohybridMbrImage}"
 fi
 
 if test -n "$efiBootable"; then
-    bootFlags="$bootFlags -eltorito-alt-boot -e $efiBootImage -no-emul-boot"
+    efiBootFlags="-eltorito-alt-boot
+                  -e $efiBootImage
+                  -no-emul-boot
+                  -isohybrid-gpt-basdat"
 fi
 
 touch pathlist
@@ -74,18 +83,41 @@ for ((n = 0; n < ${#objects[*]}; n++)); do
     fi
 done
 
-# !!! what does this do?
+# Escape filenames that contain '='.
+# TODO: Handle this properly. This fails for filenames
+#       that contain multiple '=' symbols.
 cat pathlist | sed -e 's/=\(.*\)=\(.*\)=/\\=\1=\2\\=/' | tee pathlist.safer
 
 
 mkdir -p $out/iso
-genCommand="genisoimage -iso-level 4 -r -J $bootFlags -hide-rr-moved -graft-points -path-list pathlist.safer ${volumeID:+-V $volumeID}"
-if test -z "$compressImage"; then
-    $genCommand -o $out/iso/$isoName
-else
-    $genCommand | bzip2 > $out/iso/$isoName.bz2
+
+xorriso="xorriso
+ -as mkisofs
+ -iso-level 3
+ -volid ${volumeID}
+ -appid nixos
+ -publisher nixos
+ -graft-points
+ -full-iso9660-filenames
+ ${isoBootFlags}
+ ${usbBootFlags}
+ ${efiBootFlags}
+ -r
+ -path-list pathlist.safer
+ --sort-weight 0 /
+ --sort-weight 1 /isolinux" # Make sure isolinux is near the beginning of the ISO
+
+$xorriso -output $out/iso/$isoName
+
+if test -n "$usbBootable"; then
+    echo "Making image hybrid..."
+    isohybrid --uefi $out/iso/$isoName
 fi
 
+if test -n "$compressImage"; then
+    echo "Compressing image..."
+    bzip2 $out/iso/$isoName
+fi
 
 mkdir -p $out/nix-support
 echo $system > $out/nix-support/system