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/ostree-grub-generator

@@ -0,0 +1,101 @@
+#!/bin/sh
+
+# To use a custrom script for generating grub.cfg, set the --with-grub2-mkconfig=no
+# configure switch when configuring and building OSTree.
+#
+# This script is called by ostree/src/libostree/ostree-bootloader-grub2.c whenever
+# boot loader configuration file needs to be updated. It can be used as a template
+# for a custom grub.cfg generator. What to consider when writing a custom grub.cfg
+# generator:
+#
+#   - The populate_menu() function converts boot loader entries as defined by
+#   https://www.freedesktop.org/wiki/Specifications/BootLoaderSpec/ into GRUB2
+#   menuentry sections. This is the core logic that is required by OSTree
+#   based system.
+#
+#   - Embedded systems: Be aware that this script is executed not only on a host machine by OS
+#   installer, but also on a target device, thus think about shell portability. A target device
+#   for example might be using busybox with a limited shell.
+#
+# Feel free to edit this script to fit your requirements.
+
+set -e
+
+script=$(basename ${0})
+# Atomically safe location where to generete grub.cfg when executing system upgrade.
+new_grub2_cfg=${2}
+entries_path=$(dirname $new_grub2_cfg)/entries
+
+read_config()
+{
+    config_file=${entries_path}/${1}
+    title=""
+    initrd=""
+    options=""
+    linux=""
+
+    while read -r line
+    do
+        record=$(echo ${line} | cut -f 1 -d ' ')
+        value=$(echo ${line} | cut -s -f2- -d ' ')
+        case "${record}" in
+            "title")
+                title=${value}
+                ;;
+            "initrd")
+                initrd=${value}
+                ;;
+            "linux")
+                linux=${value}
+                ;;
+            "options")
+                options=${value}
+                ;;
+        esac
+    done < ${config_file}
+
+    if [ -z "${title}" ]; then
+        title="(Untitled)"
+    fi
+}
+
+populate_menu()
+{
+    boot_prefix="${OSTREE_BOOT_PARTITION}"
+    for config in $(ls ${entries_path}); do
+        read_config ${config}
+        menu="${menu}menuentry '${title}' {\n"
+        menu="${menu}\t linux ${boot_prefix}${linux} ${options}\n"
+        menu="${menu}\t initrd ${boot_prefix}${initrd}\n"
+        menu="${menu}}\n\n"
+    done
+    # The printf command seems to be more reliable across shells for special character (\n, \t) evaluation
+    printf "$menu" >> ${new_grub2_cfg}
+}
+
+populate_warning()
+{
+cat >> ${new_grub2_cfg} <<EOF
+# This file was generated by ${script}. Do not modify the generated file - all changes will
+# be lost the next time file is regenerated. For more details refer to the ${script} script.
+EOF
+}
+
+populate_header()
+{
+cat >> ${new_grub2_cfg} <<EOF
+serial --unit=0 --speed=115200 --word=8 --parity=no --stop=1
+default=boot
+timeout=10
+
+EOF
+}
+
+generate_grub2_cfg()
+{
+    populate_warning
+    populate_header
+    populate_menu
+}
+
+generate_grub2_cfg