boot: bootflow_menu: fix crash for EFI BOOTMGR global bootmeth

The global bootmeths don't set the dev in bootflow struct which means
the dev_get_parent(bflow->dev) triggers a NULL-pointer dereference and
crash U-Boot.

So before trying to handle a bootflow, check that the associated
bootmeth isn't global, otherwise skip it.

Suggested-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/include/bootflow.h b/include/bootflow.h
index 080ee85..6affc5e 100644
--- a/include/bootflow.h
+++ b/include/bootflow.h
@@ -63,7 +63,8 @@
  *
  * @bm_node: Points to siblings in the same bootdev
  * @glob_node: Points to siblings in the global list (all bootdev)
- * @dev: Bootdev device which produced this bootflow
+ * @dev: Bootdev device which produced this bootflow, NULL for flows created by
+ *      BOOTMETHF_GLOBAL bootmeths
  * @blk: Block device which contains this bootflow, NULL if this is a network
  *	device or sandbox 'host' device
  * @part: Partition number (0 for whole device)