fix(common): Disable parallel_emerge by default for most commands.

I would like to phase out parallel_emerge so disable it for all commands
other than build_image which is the only one that shows a noticeable
benefit from it (~2 min with --fast, ~3 min with --nofast).
fix(make_chroot): Fix SDK setup w/ empty passwd and group files

New baselayout 3 based SDKs define system files in separate read-only
files so the normal group and passwd files don't exist yet.
Merge pull request #179 from marineam/nofast

Disable parallel_emerge, fix using SDKs w/ new baselayout