Add serial port example with mbed framework

examples/mbed/mbed-serial/src/main.cpp

@@ -0,0 +1,10 @@
+#include "mbed.h"
+
+Serial pc(USBTX, USBRX); // tx, rx
+
+int main() {
+    pc.printf("Hello World!\n\r");
+    while(1) {
+        pc.putc(pc.getc() + 1); // echo input back to terminal
+    }
+}