pbio/main: Use REPL for autostart.

This is the most convenient for testing on builds without full pbsys.

lib/pbio/sys/main.c

@@ -64,13 +64,13 @@ int main(int argc, char **argv) {
     pbio_init();
     pbsys_init();
 
-    #if PBSYS_CONFIG_USER_PROGRAM_AUTO_START
-    pbsys_main_program_request_start(0);
-    #endif
-
     // Keep loading and running user programs until shutdown is requested.
     while (!pbsys_status_test(PBIO_PYBRICKS_STATUS_SHUTDOWN_REQUEST)) {
 
+        #if PBSYS_CONFIG_USER_PROGRAM_AUTO_START
+        pbsys_main_program_request_start(PBIO_PYBRICKS_USER_PROGRAM_ID_REPL);
+        #endif
+
         // REVISIT: this can be long waiting, so we could do a more efficient
         // wait (i.e. __WFI() on embedded system)
         while (pbio_do_one_event()) {