Added serial_break_clear/serial_break_set

Fixes #24

Author: matthewelse

libraries/mbed/targets/hal/TARGET_NXP/TARGET_LPC11XX/serial_api.c

@@ -274,3 +274,11 @@ void serial_clear(serial_t *obj) {
 void serial_pinout_tx(PinName tx) {
     pinmap_pinout(tx, PinMap_UART_TX);
 }
+
+void serial_break_clear(serial_t *obj) {
+    obj->uart->LCR &= ~(1 << 6);
+}
+
+void serial_break_set(serial_t *obj) {
+    obj->uart->LCR |= 1 << 6;
+}