UARTSerial: Avoid readable() ambiguity

UARTSerial inherits both FileHandle::readable() [public] and
SerialBase::readable() [private], so calling readable() on a UARTSerial
object produces an ambiguous member error.

Add using declarations to direct towards the FileHandle versions of
readable and writable.

There's currently no ambiguity for writable, as SerialBase uses the
spelling 'writeable', but add a using directive for that anyway, in case
SerialBase gains 'writable' later.

drivers/UARTSerial.h

@@ -56,6 +56,12 @@ public:
      */
     virtual short poll(short events) const;
 
+    /* Resolve ambiguities versus our private SerialBase
+     * (for writable, spelling differs, but just in case)
+     */
+    using FileHandle::readable;
+    using FileHandle::writable;
+
     /** Write the contents of a buffer to a file
      *
      *  @param buffer   The buffer to write from