sysroot: Add ostree_sysroot_write_origin_file() API

We want to allow admins to change the origin file without doing a new
deployment, so this will be part of a future "admin set-origin"
command.

src/libostree/ostree-sysroot.h

@@ -61,6 +61,12 @@ gboolean ostree_sysroot_cleanup (OstreeSysroot       *self,
                                  GCancellable        *cancellable,
                                  GError             **error);
 
+gboolean ostree_sysroot_write_origin_file (OstreeSysroot         *sysroot,
+                                           OstreeDeployment      *deployment,
+                                           GKeyFile              *new_origin,
+                                           GCancellable          *cancellable,
+                                           GError               **error);
+
 gboolean ostree_sysroot_get_repo (OstreeSysroot         *self,
                                   OstreeRepo           **out_repo,
                                   GCancellable          *cancellable,