docs/Makefile: use conditional assignment on sphinx variables

If 'makefile' variables are expected to be overwritten by environment
and/or command line, it is better to use conditional assignment ('?=')
so it clearly indicates the variable intention and usage.

Change-Id: I890f71fe4bb26b5777b49e9904a1489ca8fad498
Signed-off-by: Leonardo Sandoval <leonardo.sandoval@linaro.org>
1 file changed