Add option for bootloader to be TFTP client

Were I to design this bootloader from scratch, I would have the device connect to a server with a known IP during boot, check a version, and update if the version is different. I'm not sure how much work that would be, just a thought.

Another benefit is that we could have the bootloader run the check every boot, and the end user application doesn't have to use the EthernetReset server.