restore previous build uuid even if checkpoints are disabled
make workdir parameter optional
introduce the "include_steps" parameter that allows us to specify one or more directories where to look for steps
Get /etc/mtab available in chroot
record the bash history and the environment variables for each context
Installing the bootloader (extlinux) depends on the bootstrap method (nbd..)
Rename explicitly some export and bootstrap steps.

Prepare appliance and save appliance are dependent from each other...