Split installation into stages

[Alexander--]

If something fails, whole installation have to be started again. Several such occurences can eat up lots of traffic and kill one's SD card. It would be great to split the process in several transactional stages:

• pre-root actions
• getting root
• setting up host system
• preparing Debian partition
• downloading packages and creating tarboll with them on SD card (see corresponding debootstrap option)
• debootstraping from that tarboll
• finishing installation.

Every step should check if previous one was succesfull. User should be able to manually start from any step.

[eighthave]

This would be great to have, its mostly a matter of someone finding the time to implement and test this