ports/esp32: Add `MICROPY_BOARD_FROZEN_BOOT_FILE`.

This adds the option to execute custom frozen boot module by defining the
macro `MICROPY_BOARD_FROZEN_BOOT_FILE`. This feature is already
available in stm32 and mimxrt ports.

Signed-off-by: Carlosgg <carlosgilglez@gmail.com>

ports/esp32/main.c

@@ -137,6 +137,12 @@ soft_reset:
 
     // run boot-up scripts
     pyexec_frozen_module("_boot.py", false);
+
+    // Run optional frozen boot code.
+    #ifdef MICROPY_BOARD_FROZEN_BOOT_FILE
+    pyexec_frozen_module(MICROPY_BOARD_FROZEN_BOOT_FILE, false);
+    #endif
+
     int ret = pyexec_file_if_exists("boot.py");
     if (ret & PYEXEC_FORCED_EXIT) {
         goto soft_reset_exit;