doc/Makefile: Fix build dir setting

The commit 4d8da8ed ("Docs: Update sphinx targets with the build directory")
introduces an additional variable intending to allow an user to specify
a different build directory. Since the variable is not writable from the
outside and also the Docker container used to build doc.coreboot.org
calls the Makefile with `BUILDDIR` instead of `SPHINXDIR`, building the
documentation within the container doesn't work anymore.

Thus, change the variable name to `BUILDDIR` and make it writable.

Steps to reproduce:

  cd util/docker
  make doc.coreboot.org
  make docker-build-docs

Change-Id: Ibc44134cf1996592597252aeb9dcf7ffb3378ee3
Signed-off-by: Felix Singer <felixsinger@posteo.net>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/75893
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nicholas Chin <nic.c3.14@gmail.com>
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-by: Martin L Roth <gaumless@gmail.com>

0 files changed, 0 insertions, 0 deletions