skeleton: optionally wait for network interfaces to appear

skeleton: optionally wait for network interfaces to appear

This patch has same purpose than 49964858f45d2243c513e6d362e992ad89ec7a45:

  On some machines, the network interface is slow to appear. For example,
  on the Raspberry Pi, the network interface eth0 is an ethernet-over-USB,
  and our standard boot process is too fast, so our network startup script
  is called before the USB bus is compeltely enumerated, thus it can't
  configure eth0.

  Closes #8116.

However, wait-delay hook is enabled only if wait-delay property appears
in /etc/network/interfaces. This patch enable it automaticaly when
interface is configured through DHCP at bootup. But, if user choose
to write /etc/network/interface himself, he have to explicitly
set wait-delay.

Signed-off-by: Jérôme Pouiller <jezz@sysmic.org>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>