fix(sys-kernel/coreos-kernel): Remove root= from built in flags.

This is no longer required and is incorrect for usr images.
fix(coreos-base/coreos): Add glibc as explicit run-time dependency

Turns out the only package that was pulling in glibc as a run-time
dependency in our images was pam. This breaks when pam is disabled. :)
fix(profiles): Disable pam in -usr images.

pam complicates our tiny little OS, I don't think we actually have a
need for pam so instead of fixing it to move configs to /usr just
disable it. If someone comes up with a need we can fix pam later.
bump(sys-kernel/bootengine): PXE related updates
fix(sys-apps/shadow): Install configs to /usr and symlinks to /etc

This ensures logging in still works after booting with an empty /etc.

Remove some pkg_pre/postinst fixups that we don't need.
bump(coreos-base/coretest): Include latest fixes
bump(coreos-base/coreos-init): Update for PXE/PAM fixes.
Merge pull request #475 from marineam/usr

PXE and other updates