esptool: (re)populate the secure boot signing key path

For project with reduced component list, secure boot signing key
path was not getting correctly. This change sets the secure boot
signing key path explicitly based on the relevant config option.

Closes https://github.com/espressif/esp-idf/issues/9578
Close IDFGH-8075

components/esptool_py/project_include.cmake

@@ -140,6 +140,8 @@ endif()
 if(NOT BOOTLOADER_BUILD AND CONFIG_SECURE_SIGNED_APPS)
     if(CONFIG_SECURE_BOOT_BUILD_SIGNED_BINARIES)
         # for locally signed secure boot image, add a signing step to get from unsigned app to signed app
+        get_filename_component(secure_boot_signing_key "${CONFIG_SECURE_BOOT_SIGNING_KEY}"
+            ABSOLUTE BASE_DIR "${project_dir}")
         add_custom_command(OUTPUT "${build_dir}/.signed_bin_timestamp"
             COMMAND ${ESPSECUREPY} sign_data --version ${secure_boot_version} --keyfile ${secure_boot_signing_key}
                 -o "${build_dir}/${PROJECT_BIN}" "${build_dir}/${unsigned_project_binary}"