Use all available cores to speed up building the documentation.
As $(SPHINXOPTS) has no default in `Makefile.sphinx`, we can
communicate it to all sub-makes via `export`.
Change-Id: I25996f17348505722f3489a15a975de620331b5a
Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Nico Huber <nico.h@gmx.de>
Original-reviewed-on: https://review.coreboot.org/c/coreboot/+/77363
Reviewed-on: https://review.coreboot.org/c/coreboot/+/77452
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin L Roth <gaumless@gmail.com>
7de2fa3c7f
