[Tails-dev] Heads up: update your build system

Hi,

I've just merged !478 into testing & devel, so you need to update your
build system accordingly to replace vmdebootstrap with vmdb2.

tl;dr: install vmdb2 from testing/sid

For the longer and more helpful version, see
wiki/src/contribute/build.mdwn on the testing branch.

Cheers!