Feature: Added non-blocking serial break/unbreak functions

Current serial implementation has a send_break() command which
sends a break command on the UART for a fixed amount of time.

Added functions to allow users to send a break in a non-blocking
fashion, as well as for a user-specified amount of time.

drivers/SerialBase.cpp

@@ -118,6 +118,20 @@ int SerialBase::_base_putc(int c)
     return c;
 }
 
+void SerialBase::set_break()
+{
+    lock();
+    serial_break_set(&_serial);
+    unlock();
+}
+
+void SerialBase::clear_break()
+{
+    lock();
+    serial_break_clear(&_serial);
+    unlock();
+}
+
 void SerialBase::send_break()
 {
     lock();

drivers/SerialBase.h

@@ -137,6 +137,16 @@ public:
         attach(callback(obj, method), type);
     }
 
+    /** Generate a break condition on the serial line
+     *  NOTE: Clear break needs to run at least one frame after set_break is called
+     */
+    void set_break();
+
+    /** Clear a break condition on the serial line
+     *  NOTE: Should be run at least one frame after set_break is called
+     */
+    void clear_break();
+
     /** Generate a break condition on the serial line
      */
     void send_break();