Sometimes an element needs packages installed so that it can
perform tasks but those package are not appropriate for the
final image content. Add a "build-only" flag to package-install-squash
which will cause package to be installed at the beginning of the
phase and then uninstalled at the end of the phase.
Change-Id: Ie01b795991710c93f6b669c8f14b57eb4412c1d5