doc: strip zephyr root from generated docs

Signed-off-by: Anas Nashif <anas.nashif@intel.com>
diff --git a/doc/Makefile b/doc/Makefile
index 48aa35e..8dc3c9f 100644
--- a/doc/Makefile
+++ b/doc/Makefile
@@ -26,7 +26,7 @@
 .PHONY: help clean html dirhtml singlehtml pickle json htmlhelp qthelp devhelp epub latex latexpdf text man changes linkcheck doctest coverage gettext
 
 doxy-code:
-	$(Q)doxygen zephyr.doxyfile 2>&1 | tee doc.log
+	$(Q)(cat zephyr.doxyfile ; echo "STRIP_FROM_PATH=${ZEPHYR_BASE}" ) | doxygen -  2>&1 | tee doc.log
 
 doxy: doxy-code