doc: conf: specify which variable is used for output directory

This is needed so that it can be re-used by other Doxygen configuration
entries.

Signed-off-by: Gerard Marull-Paretas <[email protected]>

Author: gmarull

doc/conf.py

@@ -185,6 +185,7 @@
 doxyrunner_outdir = ZEPHYR_BUILD / "doxygen"
 doxyrunner_fmt = True
 doxyrunner_fmt_vars = {"ZEPHYR_BASE": str(ZEPHYR_BASE), "ZEPHYR_VERSION": version}
+doxyrunner_outdir_var = "DOXY_OUT"
 
 # -- Options for Breathe plugin -------------------------------------------