bootstrap logic changes

[DJ Delorie]

I've finally gotten around to checking in the "make bootstrap" changes
I've been, well, hanging onto.  Changes:

* Stage targets renamed from stage_[abcdef] to, say, stage1_build,
  stage2_copy, etc.

* Boostrapping prints a "what to do next" message when it's done.

* "make unstage[1234]" will undo a "make stage[1234]"

* "make restage[1234]" will undo and rebuild stage [1234].

* "make quickstrap" redoes the current stage with the right flags.

* "make cleanstrap" does "make clean; make bootstrap".

* "make bubblestrap" does an incremental full bootstrap.  Basically,
  it unstages each stage in turn and rebuilds/restages them all.