Bring nanobsd.sh up to date.

Bring nanobsd.sh up to date.

MFC:
r212938: Support new variable NANO_LABEL.
r212990: Make the labels match the device name that's mounted, not just the
   slice they are on.
r214955:
  - Set -x flag when executing customisation scripts to aid in debugging them.
  - Use KERNCONFDIR with KERNCONF instead of copying the kernel config into
    the source tree so included kernel configs work.
  - Put more stuff in the _.bk/_.ik log file, not just make statements.
  - Add the kernel config name to the pprint during kernel installation.
  - Add NANO_MODULES providing a list of modules to build and install.
r215069: Document NANO_CFGDIR and NANO_DATADIR
r215070: Build make.conf when the world is not selected to build, but the
  kernel is.
r215081: Insulate the nanobsd build from the current system by opting out of
  the SRCCONF processing.
r216144: _WITHOUT_SRCCONF has too much baggage.  Instead, use the simpler
  SRCCONF=/dev/null.
r216145:
  - Mount the device async when we're doing the copy.
  - Create a sparse file instead of a fully zerod one.  This trades the
    possibiltiy of running out of space during the build for the speed
    gain not having do write all those zeros...
r216928: Put in the other half of the SRCCONF patch.
r216929: Bump the media size from approx 600MB to approx 750MB.  The great
  hob-nailed tennis shoe of progress demands it!
r220091: Use ${NANO_WORLDDIR}/var/empty as copy source since
  it has no schg flag set.
r221850: Copy symbolic links as files rather than recreating the links.
r221851: Implement -f to inhibit copying s1 partition out for speed.
r221852: Add ${} around variable dereference...
r221856: If there's no package directory, don't try to install packages from
  it.  Instead, report that 0 packages are reported.
r221877: Restore BOOT2CFG, accidentally removed in r212938.
r222535: Don't need (and can't use) -L to copy links here.

Parts of the MFC's requested by: jpaetzel@

git-svn-id: svn://svn.freebsd.org/base/stable/8@225923 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f