ifupdown: install missing directories

There are four directories in which scripts can be placed which will
always be run for any interface during certain phases of ifup and ifdown
commands:
/etc/network/if-pre-up.d/
/etc/network/if-up.d/
/etc/network/if-down.d/
/etc/network/if-post-down.d/

Even if there are no scripts in these directories, ifup and ifdown
commands will also search these directories by using run-parts command.

Install these directories to fix the following runtime errors:
$ cat /etc/network/interfaces
auto lo
iface lo inet loopback
$ ifdown lo
ifdown: interface lo not configured
$ ifup lo
run-parts: failed to open directory /etc/network/if-up.d: No such file or directory
ifup: failed to bring up lo

Signed-off-by: Yi Zhao <yi.zhao@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>