git.cmake: print BUILD_VERSION always, simplify and fix error message

In an ideal world, good CIs make it very clear what exact git versions
are getting built. However:

- Zephyr is (re-)used in many projects and they cannot all be expected
  to have ideal CI.

- CI with multiple git repos is complex (#34713) which makes the world
  an even less ideal place: much more chance for some git versions to be
  missing.

- Many developers don't realize that Github and other CIs do not test
  pull/12345/head but the moving target pull/12345/merge instead. While
  not resolving pull/12345/merge completely (maybe another day), this
  commit provides at least evidence that pull/12345/head is NOT the
  commit tested.

So the addition of the following line in the logs is a very small price
to pay that can save enormous amounts of time when trying to understand
some obscure build failures.

-- BUILD_VERSION=zephyr-v2.5.0-2957-g6230b5bb66bc

Note this obviously does not provide any git information when
BUILD_VERSION is overriden but it does not hurt either in this case:
knowing BUILD_VERSION was overriden is also useful.

The "BUILD_VERSION is left undefined" message was just wrong in
the (unlikely) case `git describe` printed something while also
failing. Remove it; it's so much simpler to just print $BUILD_VERSION
and give the direct, unfiltered information.

Note this simplification is also a partial revert of 1b80f00f
which threw the entire git warnings "baby" with some obscure duplicate
1.13.0 "bathwater" that is not relevant any more and that I guess barely
anyone noticed even at the time.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>