[GreatFET] Updating cloned github GreatFet

[Carl Wall]

Was wondering if
https://github.com/greatscottgadgets/greatfet/wiki/Getting-Started-with-Firmware-Development
could be updated with the proper procedure to update the local repository to
keep in step with code changes on github, since it pulls in other libraries.


See the bus pirate is getting a refresh, moving to arm and the libopencm3
library

http://dangerousprototypes.com/blog/2018/09/11/updated-bus-pirate-v3-x-concept-design/

http://dangerousprototypes.com/blog/2018/09/13/prototype-bus-pirate-next-gen-v1/

Cheers Carl