Add a target to check broken external links in the Documentation

Documentation shouldn't have broken links.
sphinx linkcheck builder scans all documents for external links, tries
to open them with urllib2, and writes an overview which ones are broken
and redirected to standard output and to output.txt in the output
directory.
Reviewed-by: NJani Nikula <jani.nikula@intel.com>
Tested-by: NJani Nikula <jani.nikula@intel.com>
Signed-off-by: NRémy Léone <remy.leone@gmail.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/DocBook/Makefile

@@ -71,6 +71,7 @@ installmandocs: mandocs
 # no-op for the DocBook toolchain
 epubdocs:
 latexdocs:
+linkcheckdocs:
 
 ###
 #External programs used

Documentation/Makefile.sphinx

@@ -69,6 +69,9 @@ quiet_cmd_sphinx = SPHINX  $@ --> file://$(abspath $(BUILDDIR)/$3/$4)
 htmldocs:
 	@+$(foreach var,$(SPHINXDIRS),$(call loop_cmd,sphinx,html,$(var),,$(var)))
 
+linkcheckdocs:
+	@$(foreach var,$(SPHINXDIRS),$(call loop_cmd,sphinx,linkcheck,$(var),,$(var)))
+
 latexdocs:
 	@+$(foreach var,$(SPHINXDIRS),$(call loop_cmd,sphinx,latex,$(var),latex,$(var)))
 
@@ -117,6 +120,7 @@ dochelp:
 	@echo  '  pdfdocs         - PDF'
 	@echo  '  epubdocs        - EPUB'
 	@echo  '  xmldocs         - XML'
+	@echo  '  linkcheckdocs   - check for broken external links (will connect to external hosts)'
 	@echo  '  cleandocs       - clean all generated files'
 	@echo
 	@echo  '  make SPHINXDIRS="s1 s2" [target] Generate only docs of folder s1, s2'

Documentation/media/Makefile

@@ -103,6 +103,7 @@ html: all
 epub: all
 xml: all
 latex: $(IMGPDF) all
+linkcheck:
 
 clean:
 	-rm -f $(DOTTGT) $(IMGTGT) ${TARGETS} 2>/dev/null

Makefile

@@ -1444,7 +1444,7 @@ $(help-board-dirs): help-%:
 
 # Documentation targets
 # ---------------------------------------------------------------------------
-DOC_TARGETS := xmldocs sgmldocs psdocs latexdocs pdfdocs htmldocs mandocs installmandocs epubdocs cleandocs
+DOC_TARGETS := xmldocs sgmldocs psdocs latexdocs pdfdocs htmldocs mandocs installmandocs epubdocs cleandocs linkcheckdocs
 PHONY += $(DOC_TARGETS)
 $(DOC_TARGETS): scripts_basic FORCE
 	$(Q)$(MAKE) $(build)=scripts build_docproc build_check-lc_ctype