busybox 1.20.x: add if-post-up.d and if-pre-down.d to device table

Busybox 1.20.0 add functionality to execute scripts from if-post-up.d
and if-pre-down.d, so add the missing directories to the device table to
get them created and avoid unnecessary warnings if not.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>