Use upstream vmdebootstrap

When i try to build an image for the devkit on my Ubuntu 18.04 host, it fails with the following error:

ERROR: Only UEFI is supported on arm64

A quick Google loo.kup shows that the reason is that vmdebootstrap doesn't support building arm64 image without uefi, which is a known issue[1] Turned out that the vmdebootstrap version used in the docker image is patched[2] to bypass this, so after I patched my host's vmdebootstrap version as well, the image builder works, although it's a rather hackish workaround. I think this should be at least documented (gonna send a PR soon to cover this in the ReadMe), or migrating away from vmdebootstrap to vmdb2, as vmdebootstrap is abandonware anyway. Is there any plan to do this migration?

[1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=845439

[2] e28ecaf5

EDIT: The proper solution is to use an upstream version of vmdebootstrap, like described above.