Makefile.inc1: allow real-update-packages to be called independently

[svmhdvn]

To perform an incremental update of a pkgbase repo, you would call 'make update-packages', which will stage, create, and incrementally choose newer package versions to sign as part of a pkg repo. However, this forces you to stage the kernel and source packages along with the world packages. For a jail-only installation of FreeBSD, these packages are generally not required.

This patch separates the 'update-packages' target from the 'real-update-packages' target to allow choosing world, kernel, and/or source individually at the command line like so:

Jail-only installation:

make [...] -j$(nproc) buildworld
make [...] PKG_VERSION='15.snap<date>' \
    stage-packages-world create-packages-world \
    real-update-packages

Other minor changes are included to fix the PKG_VERSION variable definition logic and order of operations.