$ ifup br0.6
would go throup _none_up of links.defn and never call the
set_environ for any of the files. Check for environ being
null and then call the set_environ function to initialize
the PATH. This is changing the semantic to allow the iface,
mode and phase to be NULL. I tried to add proper null checks
--exit-on-error is not supported by the busybox run-parts
and the easiest way is to make the ignore-errors option
the default.
I didn't test the change on the device but is looks sound.