Teensy 4.1 demo: start USB serial shell after control line state

Without this change, the ChibiOS shell prompt does not show up reliably on
Linux, and possibly other operating systems.

For a detailed walk-through covering the individual problems and causes, see:
https://michael.stapelberg.ch/posts/2021-04-27-linux-usb-virtual-serial-cdc-acm/

I verified that this approach works with:

1. Linux 5.10
2. Windows 10
3. macOS Big Sur on a MacBook Air M1

demos/MIMXRT1062/RT-TEENSY4_1/main.c

@@ -15,6 +15,7 @@
 */
 
 #include "ch.h"
+#include "chthreads.h"
 #include "hal.h"
 #include "ch_test.h"
 #include "rt_test_root.h"
@@ -92,6 +93,8 @@ void printf_debug(const char *format, ...)
   va_end(args);
 }
 
+semaphore_t scls;
+
 /*
  * Application entry point.
  */
@@ -107,6 +110,8 @@ int main(void) {
   halInit();
   chSysInit();
 
+  chSemObjectInit(&scls, 0);
+  
   /*
    * Activates MYSERIAL with 115200 baud.
    */
@@ -152,6 +157,11 @@ int main(void) {
 
   while (true) {
     if (SDU1.config->usbp->state == USB_ACTIVE) {
+      // Wait until sduRequestsHook CDC_SET_CONTROL_LINE_STATE happens, then
+      // sleep for a certain time to give the app a chance to configure flags.
+      chSemWait(&scls);
+      chThdSleepMilliseconds(100);
+
       chprintf((BaseSequentialStream*)MYSERIAL, "Starting serial-over-USB CDC Shell\r\n");
       thread_t *shelltp = chThdCreateFromHeap(NULL, SHELL_WA_SIZE, "shell", NORMALPRIO + 1, shellThread, (void *)&shell_cfg1);
       chThdWait(shelltp); /* Waiting termination.             */

demos/MIMXRT1062/RT-TEENSY4_1/usbcfg.c

@@ -316,13 +316,28 @@ static void sof_handler(USBDriver *usbp) {
   osalSysUnlockFromISR();
 }
 
+extern semaphore_t scls;
+
+bool requests_hook(USBDriver *usbp) {
+  const bool result = sduRequestsHook(usbp);
+
+  if ((usbp->setup[0] & USB_RTYPE_TYPE_MASK) == USB_RTYPE_TYPE_CLASS &&
+      usbp->setup[1] == CDC_SET_CONTROL_LINE_STATE) {
+    osalSysLockFromISR();
+    chSemResetI(&scls, 0);
+    osalSysUnlockFromISR();
+  }
+
+  return result;
+}
+
 /*
  * USB driver configuration.
  */
 const USBConfig usbcfg = {
   usb_event,
   get_descriptor,
-  sduRequestsHook,
+  requests_hook,
   sof_handler
 };