Add scripts for building the net tests rootfs.

These scripts enable the bootstrapping of the Debian images used by the
net tests. The default image type, and only recommended one at this
time, is based on Debian Stretch (9.4). The stretch image now contains a
copy of the AOSP version of iptables (which has full bpf support).

Although the knowledge of exactly how the original Debian Wheezy (7.0)
image was built has been lost, this change also includes a script which
builds a compatible image.

Both images support running with their native init system with UML and
QEMU, or booting directly into the net_test.sh script.

Bug: 74373492
Bug: 79694505
Change-Id: I497fac43b9cef6551efee1cc2134075d16ee2481
Signed-off-by: Alistair Strachan <astrachan@google.com>