fix(80gptprio): Gut most of gptprio, make it work for root and usr

The usrgptprio module got too complicated. Simplify things down to a
module that does nothing but parse command line args and possibly load a
kernel and use kexec. Remove retry/fallback logic by simply rebooting
instead. After loading the kernel leave the actual kexecing to systemd.
add(80setup-root): Move fs setup logic to new module

This module only runs on /usr images. The old manual symlink creation
logic has been replaced by calling systemd-tmpfiles with it's new shiny
--root argument. Only rules from baselayout files are evaluated to avoid
polluting the root file system just in case some places like /home have
been configured to be mount points.
remove(80usrgptprio): Replaced by new gptprio and setup-root modules
Merge pull request #27 from marineam/reorg

rework bootengine modules