microdrive: fix a few bugs

With this done we can now do all the core bits of loading an image
zx128: kernel.def option to decide if we want to microdrive boot

Also use a temporary boot stack for this
zx128: on a panic halt and spin

This makes it much easier to debug in FUSE
mdv: turn on the motor when reading the maps - it helps
zx128, tools: Update bintomdv to upload the entire image