grub2: Generate config from pending deployment

Generate a grub2 config using the pending deployment, if a grub2
bootloader is detected in the sysroot. Allows grub2-mkconfig
to run if there are no previous deployments.

Fixes: #1774

Closes: #1831
Approved by: jlebon

src/libostree/ostree-bootloader.h

@@ -44,6 +44,7 @@ struct _OstreeBootloaderInterface
   const char *         (* get_name)               (OstreeBootloader  *self);
   gboolean             (* write_config)           (OstreeBootloader  *self,
                                                    int            bootversion,
+                                                   GPtrArray     *new_deployments,
                                                    GCancellable  *cancellable,
                                                    GError       **error);
   gboolean             (* is_atomic)              (OstreeBootloader  *self);
@@ -61,6 +62,7 @@ const char *_ostree_bootloader_get_name (OstreeBootloader  *self);
 
 gboolean _ostree_bootloader_write_config (OstreeBootloader  *self,
                                           int            bootversion,
+                                          GPtrArray     *new_deployments,
                                           GCancellable  *cancellable,
                                           GError       **error);