Etherload run&continue mode

[ki-bo]

Is your feature request related to a problem? Please describe.
Right now, etherload does not provide any possibility to return back to the running program once finished. It will always need to either do a ROM reset or jump directly into the loaded code.

Describe the solution you'd like
It would be great to use etherload to upload data to memory and then continue with the running program when done. This would include:

1. Save PC to $C0FE/FF before launching ETHLOAD
2. Have ETHLOAD save all regs (including stack pointer and location), so that previous prog can be resumed.

Describe alternatives you've considered
Keep functionality as is (already quite powerful)

Additional context
This issue was split out of #633