Introducing ostree-grub-generator

ostree-grub-generator can be used to customize
the generated grub.cfg file. Compile time
decision ostree-grub-generator vs grub2-mkconfig
can be overwritten with the OSTREE_GRUB2_EXEC
envvar - useful for auto tests and OS installers.

Why this alternative approach:

1) The current approach is less flexible than using a
   custom 'ostree-grub-generator' script. Each system can
   adjust this script for its needs, instead of using the
   hardcoded values from ostree-bootloader-grub2.c.

2) Too much overhead on embedded to generate grub.cfg
   via /etc/grub.d/ configuration files. It is still
   possible to do so, even with this patch applied.
   No need to install grub2 package on a target device.

3) The grub2-mkconfig code path has other issues:
   https://bugzilla.gnome.org/show_bug.cgi?id=761180

Task: https://bugzilla.gnome.org/show_bug.cgi?id=762220

Closes: #228
Approved by: cgwalters

src/boot/grub2/grub2-15_ostree

@@ -0,0 +1,47 @@
+#!/bin/sh
+# 
+# Copyright (C) 2014 Colin Walters <walters@verbum.org>
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Lesser General Public License as published
+# by the Free Software Foundation; either version 2 of the licence or (at
+# your option) any later version.
+#
+# This library is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+# Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General
+# Public License along with this library; if not, write to the
+# Free Software Foundation, Inc., 59 Temple Place, Suite 330,
+# Boston, MA 02111-1307, USA.
+
+# Gracefully exit if ostree is not installed
+if ! which ostree >/dev/null 2>/dev/null; then
+    exit 0
+fi
+
+# Make sure we're in the right environment
+if ! test -n "${GRUB_DEVICE}"; then
+    echo "This script must be run as a child of grub2-mkconfig" 1>&2
+    exit 1
+fi
+
+set -e
+
+# Pick up stuff from grub's helper that we want to inject into our
+# generated bootloader configuration.  Yes, this is pretty awful, but
+# it's a lot better than reimplementing the config-generating bits of
+# OSTree in shell script.
+
+. /usr/share/grub/grub-mkconfig_lib
+
+DEVICE=${GRUB_DEVICE_BOOT:-${GRUB_DEVICE}}
+
+GRUB2_BOOT_DEVICE_ID="$(grub_get_device_id ${DEVICE})"
+export GRUB2_BOOT_DEVICE_ID
+GRUB2_PREPARE_ROOT_CACHE="$(prepare_grub_to_access_device ${DEVICE})"
+export GRUB2_PREPARE_ROOT_CACHE
+
+exec ostree admin instutil grub2-generate