USBSerial: make bool() sleep to avoid cpu hog in while !Serial

Author: facchinm

Diff for: cores/arduino/USB/PluggableUSBSerial.h

@@ -253,6 +253,8 @@ class USBSerial: public USBCDC, public ::mbed::Stream, public HardwareSerial {
     using Print::write; // pull in write(str) and write(buf, size) from Print
 
     operator bool() {
+        // call delay() to force rescheduing during while !Serial pattern
+        delay(1);
         return connected();
     }