build: Allow SPHINXOPTS from the environment

Signed-off-by: Joakim Bech <joakim.bech@linaro.org>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>
diff --git a/Makefile b/Makefile
index a251f98..73d9abd 100644
--- a/Makefile
+++ b/Makefile
@@ -2,7 +2,7 @@
 #
 #
 # You can set these variables from the command line.
-SPHINXOPTS    =
+SPHINXOPTS    ?=
 SPHINXBUILD   ?= sphinx-build
 SPHINXPROJ    = OP-TEE
 SOURCEDIR     = .