fix(bootloader): handle extra component dirs in v1 subproject

Closes https://github.com/espressif/esp-idf/issues/18651

examples/custom_bootloader/bootloader_extra_dir/extra_component4/CMakeLists.txt

@@ -0,0 +1 @@
+idf_component_register(SRCS "extra_component4.c")

examples/custom_bootloader/bootloader_extra_dir/extra_component4/extra_component4.c

@@ -0,0 +1,9 @@
+/*
+ * SPDX-FileCopyrightText: 2026 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Unlicense OR CC0-1.0
+ */
+
+void bootloader_extra_component4_marker(void)
+{
+}