fix installation of manta bootloader

Need to twiddle a special file in /sys/block to make the bootloader
block writable before writing it.

Change-Id: I3da48a2b1e5e73067e5cb64b04e1936a430bead2
diff --git a/releasetools.py b/releasetools.py
index 7945797..833c013 100644
--- a/releasetools.py
+++ b/releasetools.py
@@ -59,10 +59,13 @@
 
 def WriteBootloader(info, bootloader_img):
   common.ZipWriteStr(info.output_zip, "bootloader.img", bootloader_img)
+  common.ZipWriteStr(info.output_zip, "zero", "0")
   bl_type, bl_device = common.GetTypeAndDevice("/bootloader", info.info_dict)
 
   fstab = info.info_dict["fstab"]
 
   info.script.Print("Writing bootloader...")
+  force_ro = "/sys/block/" + bl_device.split("/")[-1] + "/force_ro"
+  info.script.AppendExtra('package_extract_file("zero", "%s");' % (force_ro,))
   info.script.AppendExtra('package_extract_file("bootloader.img", "%s");' %
                           bl_device)