release: Create /firstboot in common VM image code

[markj]

Summary

Some services, such as growfs, only run upon the first boot of an image.
The first boot is indicated by the presence of the file /firstboot,
which is unlinked after boot.

Individual cloudware types shouldn't need to create it. Do so in a
centralized place. Aside from simplifying things, this ensures that we
create a metalog entry for the firstboot file.

[cperciva]

"which is unliked after boot." -> unlinked

[emaste]

LGTM modulo the typo Colin noted. It makes sense to start by centralizing this. Eventually I'd like to go even further in formalizing an image build toolkit that gets used for VM images, nanobsd, installer, etc.