Documentation: Add make rule for sphinx-autobuild

Add livesphinx to start sphinx-autobuild

Change-Id: I9eb3217e758c2c882c759fa7ae75a39aaf1a0358
Signed-off-by: Tom Hiller <thrilleratplay@gmail.com>
Reviewed-on: https://review.coreboot.org/28210
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
diff --git a/Documentation/Makefile b/Documentation/Makefile
index b5f97d1..0fb21f5 100644
--- a/Documentation/Makefile
+++ b/Documentation/Makefile
@@ -74,3 +74,6 @@
 
 distclean: clean
 	rm -f corebootPortingGuide.pdf Kconfig.pdf
+
+livesphinx:
+	$(MAKE) -f Makefile.sphinx livehtml