haiku_loader: search for matching shortcut in supermenus

Fixes #7469.

src/system/boot/loader/menu.cpp

@@ -414,6 +414,11 @@ Menu::FindShortcut(char key) const
 		shortcut = shortcut->next;
 	}
 
+	Menu *superMenu = Supermenu();
+
+	if (superMenu != NULL)
+		return superMenu->FindShortcut(key);
+
 	return NULL;
 }
 
@@ -430,6 +435,11 @@ Menu::FindItemByShortcut(char key)
 			return item;
 	}
 
+	Menu *superMenu = Supermenu();
+
+	if (superMenu != NULL)
+		return superMenu->FindItemByShortcut(key);
+
 	return NULL;
 }