This allows use of the standard src.conf configuration for controlling whether the tree is cleaned before build or not. The default is still to clean.
Posted for initial review/comment; to commit will include an UPDATING entry. We also need to investigate these files that contain NO_CLEAN:
share/man/man5/make.conf.5
share/man/man7/build.7
share/man/man8/nanobsd.8
share/mk/local.dirdeps.mk
tools/build/beinstall.sh
tools/tinder.sh
tools/tools/nanobsd/defaults.sh
tools/tools/nanobsd/nanobsd.sh
tools/tools/nanobsd/pcengines/common.conf