This makes it possible for a VM build configuration file to pass
options to make installworld/installkernel/distribution, e.g.
WITHOUT_DEBUG_FILES=YES in order to produce smaller images.
Note that these options are only applied at install time, not at
build time (since the same build is installed into many different
VM images), so not all src.conf options are usable here.
Sponsored by: Amazon
Differential Revision: https://reviews.freebsd.org/D46506
(cherry picked from commit 81d3df02bcf5ed6e41a91fd9fbc3b81cfe809ff3)
c8d5eda2d4