Fix a problem with Debian 8 on GCE with >2TB disks

This change makes two fixes. First, it fixes a race condition on Debian
8 GCE instances which caused instances with >2TB disks to fail. Second,
it adds the required "PREREQ" header to the script, which was previously
missing and caused errors at build time. These two issues are somewhat
unrelated to each other, but debugging the first led to finding the
second.

bootstrapvz/providers/gce/assets/initramfs-tools/scripts/local-premount/gce-disable-growroot

@@ -1,5 +1,17 @@
 # Selectively disable growroot               -*- shell-script -*-
 set -e
+PREREQ=""
+prereqs()
+{
+  echo "$PREREQ"
+}
+
+case $1 in
+prereqs)
+  prereqs
+  exit 0
+  ;;
+esac
 
 message() { echo "DISABLE-GROWROOT:" "$@" ; }
 error_exit() { message "$@"; exit 1; }
@@ -12,6 +24,19 @@ error_exit() { message "$@"; exit 1; }
 # $ROOTFSTYPE - Generally empty
 # $rootmnt - Set to "/root"
 
+# According to the initramfs documentation, it's supposed to wait until
+# the disks have been attached and verified before the local-premount scripts
+# run. This does not happen, however, and failures can happen below if $ROOT
+# is referenced before the disk that it points to is attached. This allows for
+# up to 4 seconds for that to happen. In practice, it generally takes less
+# than half a second.
+for i in $(seq 1 20); do
+  if [ -e "${ROOT}" ]; then
+    break
+  fi
+  sleep 0.2
+done
+
 # Follow link to get real root location
 if [ ! -L "${ROOT}" ]; then
   real_root=${ROOT}