This is part one of a fix for cascading failure. The NuttX
nuttx repos was tagged with upstream_7.18+ and broke the tag
extraction that was expecting nuttx-M.mm
Since the git hashes were being generate by cmake it would only be
generated if the header file was not present. Simple test:
$ make aerofc-v1_default
$ touch a
$ git add a
$ git commit -m tmp
$ make aerofc-v1_default
The file build_aerofc-v1_default/build_git_version.h should have the new
hashes and the correspondent .c/c.pp files should be rebuilt, but they
aren't. The end result is that checking the version with "ver git" in
the nsh console will point to the wrong commit.
This moves the generation of the header to a separate tool and enforces
the command to be executed every time.
