Added Terminate Signal

[BjornTheProgrammer]

Description

This PR adds a termination signal when the serial connection is terminated with ctrl + c with the bytes "elf2uf2-term\n\r"

Motivation

It is annoying to either unplug and replug the mico controller while holding down the bootsel button to enter boot select mode or construct a circuit in which the ground and run pin are connected to enter boot select mode. So, it would be nice if the serial connection indicated that it would terminate. That way, it would be easier to enter boot select mode programmatically.