feat(build_image): Record directories installed in stateful_partition

During builds var_overlay is always mounted over /var. We want to do the
same at run time but we also want to ensure everything expected to be
there always does. After emerge completes gen_tmpfiles.py will scan /var
for any .keep files that were installed and records their parent
directories' permissions and ownership to /usr/lib/tmpfiles.d. On each
boot systemd will automatically recreate anything that goes missing.

This also means that going forward any ebuild that needs a directory in
/var (or anywhere else the stateful partition is bound) can simply rely
on the 'keepdir' ebuild function instead of adding things to
coreos_startup.