Documentation: Allow passing arguments into make livesphinx

It's what the doc.coreboot.org docker container is running and when
using its livehtml feature, it listens at localhost, which isn't always
desirable.

With `docker run -e SPHINXOPTS="-H $localip" ...` it now listens at
localip, which is more flexible.

Change-Id: Ia0614e57458c32169f6d614783366025e9c814b3
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/31128
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

Documentation/Makefile

@@ -52,4 +52,4 @@ distclean: clean
 	rm -f corebootPortingGuide.pdf
 
 livesphinx:
-	$(MAKE) -f Makefile.sphinx livehtml
+	$(MAKE) -f Makefile.sphinx livehtml SPHINXOPTS="$(SPHINXOPTS)"