sysutils/firstboot-pkg-upgrade: 1.1 -> 1.2

If a list of repositories has been specified, only run 'pkg update'
on those repositories; in particular, avoiding downloading the entire
FreeBSD-ports repository when we're not going to be using it allows
FreeBSD/EC2 instances (and presumably those other clouds too) to
launch roughly 5 seconds faster.

Reviewed by: ziaee
Sponsored by: Amazon
Differential Revision: https://reviews.freebsd.org/D57033

(cherry picked from commit ac4933ff6f18f126fbf9bc72f1ab9451c9355f29)