release: Create /firstboot in common VM image creation code

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 be responsible for creating it. Do
so in a centralized place. Aside from simplifying things, this ensures
that we create a metalog entry for the firstboot file.

Reviewed by: cperciva, emaste
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D52450