config/functions : allow to have a per distro functions script

This also overriding a few default functions.

config/functions

@@ -471,3 +471,8 @@ debug_strip() {
     $STRIP `find $* -type f -executable 2>/dev/null` 2>/dev/null || :
   fi
 }
+
+# Use distribution functions if any
+if [ -f "distributions/$DISTRO/config/functions" ]; then
+  . distributions/$DISTRO/config/functions
+fi