debian: fix network on Wheezy

source-directory ifupdown directive has been introduced in version
0.7.44. Wheezy comes with version 0.7.8.

Without this change, the eth0 interface will never get an IP address on
Wheezy.

Change-Id: If40ed3b82ca78280190b66c06a0a77bb71270d7d

elements/debian/root.d/08-debootstrap

@@ -59,7 +59,7 @@ else
         sudo sh -c "echo deb $DIB_DISTRIBUTION_MIRROR wheezy-backports main >> ${TARGET_ROOT}/etc/apt/sources.list"
         sudo chroot ${TARGET_ROOT} apt-get update
         cat << EOF | sudo tee -a ${TARGET_ROOT}/etc/network/interfaces
-source-directory /etc/interfaces.d
+source /etc/network/interfaces.d/*
 EOF
         apt_get_bp_extra_opts="-t wheezy-backports"
     fi