provide an option to free up TX and RX pins when using non-Serial transport

[soundanalogous]

Now that Ethernet is supported, provide an option to free up the RX and TX pins. They could be used with Firmata Serial or as normal digital I/O pins.

[soundanalogous]

stretch goal for 2.6 release

[soundanalogous]

This is complicated by the need to keep a UART available as a convenience for debugging when using Ethernet or Wi-Fi transports (especially for Wi-Fi).