fix(base_image_util): Do not auto-generate conflicting tmpfiles entries

Many directories are now handled by the new baselayout instead.
fix(build_image): Drop long unused overlays/usr/local bind mount

This was used by the old ChromeOS "dev image" scheme and does not get
mounted by default so stop shipping it.
Merge pull request #177 from marineam/baselayout

Baselayout