fix(disk_util): Install syslinux as part of filesystem formatting.

This means scripts later don't need to deal with the ESP block device
directly, they can just copy the config files into place.

build_library/disk_util

@@ -8,6 +8,7 @@ import json
 import os
 import subprocess
 import sys
+import tempfile
 import uuid
 
 # First sector we can use.
@@ -283,6 +284,19 @@ def FormatFat(part, device):
                      vfat_blocks],
                      stdout_null=True)
 
+  if 'syslinux' in part.get('features', []):
+    # The syslinux directory must exist before installing ldlinux.sys to it.
+    vfat_mount = tempfile.mkdtemp()
+    Sudo(['mount', '-t', 'vfat', device, vfat_mount])
+    try:
+      Sudo(['mkdir', os.path.join(vfat_mount, 'syslinux')])
+    finally:
+      Sudo(['umount', vfat_mount])
+      os.rmdir(vfat_mount)
+
+    Sudo(['syslinux', '-d', '/syslinux', device])
+    print "Installed SYSLINUX to %s" % part['label']
+
 
 def Format(options):
   """Writes the given partition table and initialize fresh filesystems.

build_library/legacy_disk_layout.json

@@ -12,7 +12,8 @@
         "type":"efi",
         "blocks":"262144",
         "fs_type":"vfat",
-        "mount":"/boot/efi"
+        "mount":"/boot/efi",
+        "features": ["syslinux"]
       },
       "2":{
         "label":"BOOT-B",