ifupdown: create file interfaces if not exist

It fails to run ifup if /etc/network/interfaces doesn't exist:

| ifup: couldn't open interfaces file "/etc/network/interfaces": No such
|       file or directory

Create the config file if not exist.

Signed-off-by: Kai Kang <kai.kang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>