busybox

ifupdown: improve compatibility with Debian

Set environment variable 'PHASE'.
Treat post-up and pre-down as aliases for up and down.
Uses the same logic as ifupdown.nw from ifupdown-0.6.16.
Makes it possible to execute Debian's ifupdown script for
wpa-supplicant.

Signed-off-by: Andreas Oberritter <obi@opendreambox.org>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>