Build and run in QEMU

To write

From a clean checkout to a zzsh prompt on vexpress-a15 and virt.

Build

• make BOARD=vexpress-a15, then zuzu.img and sdimg (make does not rebuild zuzu.img on its own)

Run

• make run, the serial console, quitting QEMU

Run the tests

• make smoke, what zztest checks, how long it takes