Make qemu image larger

There's no reason why we wouldn' make the qemu image 10GB or similar. It would be even better to convert the raw image to a qcow2 in the qemu case at the end of the build, we could then make it 100GB and it would grow as needed.

The first part isn't hard: make what's passed to --size a variable, default it in conf/board to the current value and use a bigger value for qemu.

The second half is one qemu-img invoction.