deploy: Find kernel/initramfs consistently from filesystem

I'm porting the deployment code to be fd-relative, but part of the
logic was using `GFile` to talk to `OstreeRepoFile` to determine the
"bootcsum" (boot config checksum) before checking out the file tree.

We can avoid having both code paths by checking out the tree first,
then looking at it on the filesystem.

src/libostree/ostree-sysroot-deploy.c

@@ -24,6 +24,7 @@
 #include <gio/gunixoutputstream.h>
 
 #include "ostree-sysroot-private.h"
+#include "ostree-deployment-private.h"
 #include "ostree-core-private.h"
 #include "ostree-linuxfsutil.h"
 #include "otutil.h"
@@ -1934,7 +1935,6 @@ ostree_sysroot_deploy_tree (OstreeSysroot     *self,
   glnx_unref_object OstreeRepo *repo = NULL;
   g_autoptr(GFile) osdeploydir = NULL;
   g_autoptr(GFile) deployment_var = NULL;
-  g_autoptr(GFile) commit_root = NULL;
   g_autoptr(GFile) tree_kernel_path = NULL;
   g_autoptr(GFile) tree_initramfs_path = NULL;
   glnx_fd_close int deployment_dfd = -1;
@@ -1960,24 +1960,6 @@ ostree_sysroot_deploy_tree (OstreeSysroot     *self,
   if (!ostree_sysroot_get_repo (self, &repo, cancellable, error))
     goto out;
 
-  if (!ostree_repo_read_commit (repo, revision, &commit_root, NULL, cancellable, error))
-    goto out;
-
-  if (!get_kernel_from_tree (commit_root, &tree_kernel_path, &tree_initramfs_path,
-                             cancellable, error))
-    goto out;
-  
-  if (tree_initramfs_path != NULL)
-    {
-      if (!checksum_from_kernel_src (tree_initramfs_path, &new_bootcsum, error))
-        goto out;
-    }
-  else
-    {
-      if (!checksum_from_kernel_src (tree_kernel_path, &new_bootcsum, error))
-        goto out;
-    }
-
   if (provided_merge_deployment != NULL)
     merge_deployment = g_object_ref (provided_merge_deployment);
 
@@ -1997,6 +1979,25 @@ ostree_sysroot_deploy_tree (OstreeSysroot     *self,
       goto out;
     }
 
+  deployment_dir = ostree_sysroot_get_deployment_directory (self, new_deployment);
+
+  if (!get_kernel_from_tree (deployment_dir, &tree_kernel_path, &tree_initramfs_path,
+                             cancellable, error))
+    goto out;
+  
+  if (tree_initramfs_path != NULL)
+    {
+      if (!checksum_from_kernel_src (tree_initramfs_path, &new_bootcsum, error))
+        goto out;
+    }
+  else
+    {
+      if (!checksum_from_kernel_src (tree_kernel_path, &new_bootcsum, error))
+        goto out;
+    }
+  
+  _ostree_deployment_set_bootcsum (new_deployment, new_bootcsum);
+
   /* Create an empty boot configuration; we will merge things into
    * it as we go.
    */