efi_client: Allow access to the parent image

This value is useful when running a binary, so provide a function to
obtain it.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/efi.h

@@ -604,6 +604,13 @@ struct efi_system_table *efi_get_sys_table(void);
  */
 struct efi_boot_services *efi_get_boot(void);
 
+/**
+ * efi_get_parent_image() - Get the handle of the parent image
+ *
+ * Return: parent-image handle
+ */
+efi_handle_t efi_get_parent_image(void);
+
 /**
  * efi_get_ram_base() - Find the base of RAM
  *