zephyr: Execute main.py file if it exists.

Adds support in the zephyr port to execute main.py if the file system is
enabled and the file exists. Existing support for executing a main.py
frozen module is preserved, since pyexec_file_if_exists() works just
like pyexec_frozen_module() if there's no vfs.

ports/zephyr/main.c

@@ -145,8 +145,8 @@ soft_reset:
     vfs_init();
     #endif
 
-    #if MICROPY_MODULE_FROZEN
-    pyexec_frozen_module("main.py");
+    #if MICROPY_MODULE_FROZEN || MICROPY_VFS
+    pyexec_file_if_exists("main.py");
     #endif
 
     for (;;) {