doc-rst: generic way to build PDF of sub-folders

This extends the method to build only sub-folders to the targets
"latexdocs" and "pdfdocs". To do so, a conf.py in the sub-folder is
required, where the latex_documents of the sub-folder are
defined. E.g. to build only gpu's PDF add the following to the
Documentation/gpu/conf.py::

  +latex_documents = [
  +    ("index", "gpu.tex", "Linux GPU Driver Developer's Guide",
  +     "The kernel development community", "manual"),
  +]

and run:

  make SPHINXDIRS=gpu pdfdocs
Signed-off-by: Markus Heiser <markus.heiser@darmarIT.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/Makefile.sphinx

@@ -71,12 +71,12 @@ ifeq ($(HAVE_PDFLATEX),0)
 	$(warning The 'xelatex' command was not found. Make sure you have it installed and in PATH to produce PDF output.)
 	@echo "  SKIP    Sphinx $@ target."
 else # HAVE_PDFLATEX
-	@$(call loop_cmd,sphinx,latex,.,latex,.)
+	@$(foreach var,$(SPHINXDIRS),$(call loop_cmd,sphinx,latex,$(var),latex,$(var)))
 endif # HAVE_PDFLATEX
 
 pdfdocs: latexdocs
 ifneq ($(HAVE_PDFLATEX),0)
-	$(Q)$(MAKE) PDFLATEX=xelatex LATEXOPTS="-interaction=nonstopmode" -C $(BUILDDIR)/latex
+	$(foreach var,$(SPHINXDIRS), $(MAKE) PDFLATEX=xelatex LATEXOPTS="-interaction=nonstopmode" -C $(BUILDDIR)/$(var)/latex)
 endif # HAVE_PDFLATEX
 
 epubdocs: