Add sysroot.bootloader repo config key

The sysroot.bootloader key configures the bootloader
that OSTree uses when deploying a sysroot. Having this key
allows specifying behavior not to use the default bootloader
backend code, which is preferable when creating a first
deployment from the sysroot (#1774).

As of now, the key can take the values "auto" or "none". If
the key is not given, the value defaults to "auto".

"auto" causes _ostree_sysroot_query_bootloader() to be used
when writing a new deployment, which is the original behavior
that dynamically detects which bootloader to use.

"none" avoids querying the bootloader dynamically. The BLS
config fragments are still written to
sysroot/boot/loader/entries for use by higher-level software.

More values can be supported in future to specify a single
bootloader, different behavior for the bootloader code, or
a list of bootloaders to try.

Resolves: #1774

Closes: #1814
Approved by: jlebon

man/ostree.repo-config.xml

@@ -109,22 +109,22 @@ Boston, MA 02111-1307, USA.
         ensure files are on stable storage when performing operations
         such as commits, pulls, and checkouts.  Defaults to
         <literal>true</literal>.</para>
-	<para>
-	  If you disable fsync, OSTree will no longer be robust
-	  against kernel crashes or power loss.
-	</para>
-	<para>
-	  You might choose to disable this for local development
-	  repositories, under the assumption they can be recreated from
-	  source.  Similarly, you could disable for a mirror where you could
-	  re-pull.
-	</para>
-	<para>
-	  For the system repository, you might choose to disable fsync
-	  if you have uninterruptable power supplies and a well tested
-	  kernel.
-	</para>
-	</listitem>
+        <para>
+          If you disable fsync, OSTree will no longer be robust
+          against kernel crashes or power loss.
+        </para>
+        <para>
+          You might choose to disable this for local development
+          repositories, under the assumption they can be recreated from
+          source.  Similarly, you could disable for a mirror where you could
+          re-pull.
+        </para>
+        <para>
+          For the system repository, you might choose to disable fsync
+          if you have uninterruptable power supplies and a well tested
+          kernel.
+        </para>
+        </listitem>
       </varlistentry>
 
       <varlistentry>
@@ -333,6 +333,42 @@ Boston, MA 02111-1307, USA.
 
   </refsect1>
 
+  <refsect1>
+    <title>[sysroot] Section Options</title>
+
+    <para>
+      Options for the sysroot, which contains the OSTree repository,
+      deployments, and stateroots.  The following entries are defined:
+    </para>
+
+    <variablelist>
+
+      <varlistentry>
+        <term><varname>bootloader</varname></term>
+        <listitem><para>Configure the bootloader that OSTree uses when
+        deploying the sysroot.  This may take the values
+        <literal>bootloader=none</literal> or <literal>bootloader=auto</literal>.
+        Default is <literal>auto</literal>.
+        </para>
+        <para>
+          If <literal>none</literal>, then OSTree will generate only BLS (Boot
+          Loader Specification) fragments in <literal>sysroot/boot/loader/entries/</literal>
+          for the deployment.
+        </para>
+        <para>
+          If <literal>auto</literal>, then in addition to generating BLS
+          fragments, OSTree will dynamically check for the existence of grub2,
+          uboot, and syslinux bootloaders.  If one of the bootloaders is found,
+          then OSTree will generate a config for the bootloader found.  For
+          example, <literal>grub2-mkconfig</literal> is run for the grub2 case.
+        </para>
+        </listitem>
+      </varlistentry>
+
+    </variablelist>
+
+  </refsect1>
+
   <refsect1>
     <title>/etc/ostree/remotes.d</title>