feature/stretch FTBFS with debootstrap 1.0.85+
Originally created by @intrigeri on #11903 (Redmine)
debootstrap 1.0.85 enabled merged-/usr by default, which breaks the build of feature/stretch, e.g. https://jenkins.tails.boum.org/view/Tails_ISO/job/build_Tails_ISO_feature-stretch/3/. It seems doable to patch our custom live-build to support both merged-/usr and non-merged-/usr.
Feature Branch: bugfix/11903-disable-merged-usr