tools/build/make.py: Keep bootstrapped bmake binary up-to-date

We currently assume that any existing bootstrapped bmake binary will
work, but this means it never gets updated as contrib/bmake is, and
similarly we won't rebuild it as and when the configure arguments given
to boot-strap change. Whilst the former isn't necessarily a huge problem
given WANT_MAKE_VERSION rarely gets bumped in Makefile, having fewer
variables is a good thing, and so it's easiest if we just always keep it
up-to-date rather than trying to do something similar to what's already
in Makefile (which may or may not be accurate, given updating FreeBSD
gives you an updated bmake, but nothing does so for our bootstrapped
bmake on non-FreeBSD). The latter is more problematic, though, and the
next commit will be changing this configuration.

We thus now add in two checks. The first is to compare MAKE_VERSION
against _MAKE_VERSION from contrib/bmake/VERSION. The second is to
record at bootstrap time the exact configuration used, and compare that
against what we would bootstrap with.

Reviewed by: arichardson, sjg
Differential Revision: https://reviews.freebsd.org/D41556