Deprecate qemu.cmake in favour of a qemu runner backend

Hi,

I suspect we want to eventually rename/alias 'flash' to 'run'. And treat qemu emulation as just another runner backend. I believe this would make it easier for us to maintain the runner and qemu infrastructure, and would make it easier for us to keep the user-interface for flashing and running consistent.

A qemu backend would also allow us to move all the symlink/fifo/pipehandling out of qemu.cmake and into a python runner backend.

It is not clear how difficult it would be to do this.