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

src/libostree/ostree-repo.c

@@ -3113,6 +3113,32 @@ reload_remote_config (OstreeRepo          *self,
   return TRUE;
 }
 
+static gboolean
+reload_sysroot_config (OstreeRepo          *self,
+                       GCancellable        *cancellable,
+                       GError             **error)
+{
+  { g_autofree char *bootloader = NULL;
+
+    if (!ot_keyfile_get_value_with_default_group_optional (self->config, "sysroot",
+                                                           "bootloader", "auto",
+                                                           &bootloader, error))
+      return FALSE;
+
+    /* TODO: possibly later add support for specifying a generic bootloader
+     * binary "x" in /usr/lib/ostree/bootloaders/x). See:
+     * https://github.com/ostreedev/ostree/issues/1719
+     * https://github.com/ostreedev/ostree/issues/1801
+     */
+    if (!(g_str_equal (bootloader, "auto") || g_str_equal (bootloader, "none")))
+      return glnx_throw (error, "Invalid bootloader configuration: '%s'", bootloader);
+
+    self->bootloader = g_steal_pointer (&bootloader);
+  }
+
+  return TRUE;
+}
+
 /**
  * ostree_repo_reload_config:
  * @self: repo
@@ -3131,6 +3157,8 @@ ostree_repo_reload_config (OstreeRepo          *self,
     return FALSE;
   if (!reload_remote_config (self, cancellable, error))
     return FALSE;
+  if (!reload_sysroot_config (self, cancellable, error))
+    return FALSE;
   return TRUE;
 }
 
@@ -6064,3 +6092,21 @@ ostree_repo_get_default_repo_finders (OstreeRepo *self)
 
   return (const gchar * const *)self->repo_finders;
 }
+
+/**
+ * ostree_repo_get_bootloader:
+ * @self: an #OstreeRepo
+ * 
+ * Get the bootloader configured. See the documentation for the
+ * "sysroot.bootloader" config key.
+ * 
+ * Returns: bootloader configuration for the sysroot
+ * Since: 2019.2
+ */
+const gchar *
+ostree_repo_get_bootloader (OstreeRepo   *self)
+{
+  g_return_val_if_fail (OSTREE_IS_REPO (self), NULL);
+
+  return self->bootloader;
+}