MFC 216487,217754,218524:

MFC 216487,217754,218524:
- Pass JFLAG as JFLAG from tinderbox to universe.
- For `make tinderbox` there is no need to print the extra commands.
- Add a new UNIVERSE_TARGET variable for 'make universe'.  If it is set,
  then that target is invoked for each architecture rather than the
  default action of building world and kernels for each architecture.
- Add a 'make toolchains' wrapper which uses UNIVERSE_TARGET to build
  toolchains for all architectures.
- Document JFLAG, MAKE_JUST_KERNELS, and MAKE_JUST_WORLDS variables for
  'make universe'.

git-svn-id: svn://svn.freebsd.org/base/stable/8@219245 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f