stm32/main: Run optional frozen module at boot.

If a board specifies a filename via MICROPY_BOARD_FROZEN_BOOT_FILE then
that will be run on start up, before the usual boot.py.

ports/stm32/main.c

@@ -564,6 +564,11 @@ soft_reset:
     // reset config variables; they should be set by boot.py
     MP_STATE_PORT(pyb_config_main) = MP_OBJ_NULL;
 
+    // Run optional frozen boot code.
+    #ifdef MICROPY_BOARD_FROZEN_BOOT_FILE
+    pyexec_frozen_module(MICROPY_BOARD_FROZEN_BOOT_FILE);
+    #endif
+
     // Run boot.py (or whatever else a board configures at this stage).
     if (MICROPY_BOARD_RUN_BOOT_PY(&state) == BOARDCTRL_GOTO_SOFT_RESET_EXIT) {
         goto soft_reset_exit;