serial: add writeBuf() and implement for USB.

This lets USB send up to 32 bytes in a frame instead of 32 one byte
frames.  Add a fallback for drivers that don't implement writeBuf().

Signed-off-by: Michael Hope <mlhx@google.com>

src/main/drivers/serial_softserial.c

@@ -479,9 +479,8 @@ const struct serialPortVTable softSerialVTable[] = {
         softSerialSetBaudRate,
         isSoftSerialTransmitBufferEmpty,
         softSerialSetMode,
-        .beginWrite = NULL,
-        .endWrite = NULL,
-  }
+        .writeBuf = NULL,
+    }
 };
 
 #endif