fixes/improvements for top of docs/build-script

Hi Jon,

This series contain the 3 patches against build-script I sent in separate:

- patch 1 solves a problem with Tumbleweed build. I opted to be verbose at the comments to properly describe what we're addressing and why;
- patch 2 solves an issue that sometimes happen on my machine while checking for CONFIG_RUST=y;
- patch 3 contains a request from Akira to allow running just the PDF step when pdfdocs target is selected.

on patch 3, I opted to not modify Makefile, as I don't see why most people would want to proceed with PDF builds when latex build fails. For those who want such feature, e. g. simulate "make -i pdfdocs" behavior, all it is needed after the change is to run:

	tools/docs/sphinx-build-wrapper latexdocs || echo "LaTeX error: $?"
	tools/docs/sphinx-build-wrapper -s pdfdocs

eventually adding some extra flags to the second command from the command line arguments below:

  --paper {,a4,letter}  Paper size for LaTeX/PDF output
  --deny-vf DENY_VF     Configuration to deny variable fonts on pdf builds
  -v, --verbose         place build in verbose mode
  -j, --jobs JOBS       Sets number of jobs to use with sphinx-build
  -i, --interactive     Change latex default to run in interactive mode

-s, --skip-sphinx-build
Skip sphinx-build step

E.g. running the second step with, for instance, with:

	tools/docs/sphinx-build-wrapper -v -s -i --deny-vf ~/.my-deny pdfdocs

to:
- assume that sphinx-build --builder latex was already executed (-s);
- provide a summary of what PDF files were built at the end (-v);
- run LaTeX in interactive mode in case of errors (-i);
- use a custom deny-list for variable fonts (--deny-vf).

If we add this at docs Makefile, and allow all possible alternatives, this would just add uneeded complexity there, IMO.

Mauro Carvalho Chehab (3):
tools/docs: sphinx-build-wrapper: fix compat with recent Tumbleweed
docs: Makefile: fix rustdoc detection tools/docs/sphinx-build-wrapper: allow skipping sphinx-build step

 Documentation/Makefile          |  4 ++-
 tools/docs/sphinx-build-wrapper | 43 ++++++++++++++++++++++++++-------
 2 files changed, 37 insertions(+), 10 deletions(-)

2.51.0