USBSerial: speedup by using non blocking read

Author: facchinm

cores/arduino/USB/PluggableUSBSerial.h

@@ -298,8 +298,16 @@ class USBSerial: public USBCDC, public ::mbed::Stream, public HardwareSerial {
     int _baud, _bits, _parity, _stop;
 
     void onInterrupt() {
-        while (rx_buffer.availableForStore() && _available()) {
-            rx_buffer.store_char(_getc());
+        uint8_t buf[256];
+        int howMany = _available();
+        uint32_t toRead;
+        if (howMany > rx_buffer.availableForStore()) {
+            howMany = rx_buffer.availableForStore();
+        }
+        receive_nb(buf, howMany, &toRead);
+        while (rx_buffer.availableForStore() && toRead > 0) {
+            rx_buffer.store_char(buf[howMany-toRead]);
+            toRead --;
         }
     }
 

cores/arduino/USB/USBSerial.cpp

@@ -72,6 +72,7 @@ void USBSerial::begin(unsigned long) {
     this->attach(::mbed::callback(this, &USBSerial::onInterrupt));
     t = new rtos::Thread(osPriorityNormal, 256, nullptr, "USBevt");
     t->start(waitForPortClose);
+    onInterrupt();
 }
 
 int USBSerial::_putc(int c)