Based on the user-provided options, define three internal variables,
_USE_PKG, _USE_PKGBASE and _USE_PORTS, to avoid having to repeat the
hard-to-read "!defined(X) || empty(X)" conditional in many places.

Add some early errors for things we know don't work, like using pkg
on non-FreeBSD platforms.

Stop ignoring errors when installing packages onto the media; if this
fails and the user is okay with that, they should set NOPKG to avoid
trying it in the first place. This avoids silently building media
which is missing expected components.