Robustness improvements

Robustness improvements: unsharing on Debian, comments in package files, and report hook.

This change adds a few convenience features to improve overall robustness of the brickstrap process:

Introduce unshare-harder variant which will prod the kernel into allowing ordinary users to unshare, which is necessary for user-unshare.pl to work on recent-ish Debian. Any changes to original kernel settings are reverted on exit. This avoids having to run the entire brickstrap with sudo/su, only the initial prodding and reverting need sudo/su privileges.
Add automatic filtering of comments in package files when $PACKAGES is assembled. This simplifies creating derivative board directories for new users (like me). Comment lines (starting with ; or #) and empty lines in package files are now ignored instead of being appended to $PACKAGES.
Add support for runnning a reporting hook. This permits the user to include a custom script which will be run at the end of sequences executed by 'run-hooks' or 'all'. Such a reporting hook is useful to aggregate and report basic information about the rootfs in one single, convenient location of the brickstrap output. For example: hostname, usernames, passwords, key signatures... This makes it easier for consumers of the rootfs to get started using their new OS as an opaque binary image.

ev3dev / brickstrap