Fix the handbook install option in bsdinstall

bsdconfig's f_package_add doesn't seem to support using the pkg repo from /etc/pkg/FreeBSD.conf, it also tries to run the commands on the installer image, not in the destination chroot

Instead, manually bootstrap pkg in the chroot, and then install the requested packages (in the chroot)

Doesn't use pkg -c, because pkg is not installed on the installer image

PR: 196250
Differential Revision: https://reviews.freebsd.org/D2026
Approved by: bapt
Sponsored by: ScaleEngine Inc.