Now that we have a top-level Makefile with a proper clean target, remove some of the intermediate build logic which was cleaning. Also remove some logic which skipped building the kernel.

This will fix certain problems with incremental builds with Jenkins.
Do not checkout source if we are building. Only checkout source if we did "make checkout".
Do not pass '-n' flag to nanobsd.sh script. It is gone.
Change "mkdir" to "mkdir -p" in a few places to fix problems with incremental builds.
Remove "hacks" for building pbiwrapper and extract-tarball.
Build and install these utilities inside the chroot, similar to how we build ports.

This cleans up some issues with incremental builds and cleaning.
Fix clean_packages() so it properly removes /var/db/pkg.

Do not look in ${NANO_WORLDDIR}/usr/local/var/db/pkg for
removing package metadata.  Instead, look in ${NANO_WORLDDIR}/var/db/pkg .
This bug has been in the script for 3 years (!!).
For now, do a 'make checkout' before executing the release target.

This is needed to preserve compatibility with the existing
release scripts.  This will be removed later.
Remove unused script.
Add a target 'make update'.

This target updates the sources in the sandbox from the SCM.
Right now, this internally calls 'make checkout', but this
may change later.
Pull out logic for checking out source tree from do_build.sh into a separate do_checkout.sh script.

In the top-level Makefile, call do_checkout.sh from the "make checkout" target.
Add a function to check that the sandbox is properly checked out.
Remove checkout from release target.  Release scripts have been modified elsewhere.
Clarify wording in error message.
Remove target which uses flags deleted from do_build.sh script.
Target is no longer needed.