Add gen_disks.sh script to modify virtual drives

We expect the repo root to contain pre-installed Alpine 3.15.4 in raw
drive file 'dummydisk.img'. This then gets copied to
output/<machine>/disk.img and the generated files are put into it in
order to be used.

Beware, this uses hardcoded paths and expects specific disk layout.
This relies on how the installation formatted the disk. While the
installation procedure is not documented anywhere, it is in essence
default setup of Alpine Linux, with minimal tweaks that should not
affect behaviour of routing (but still impede reproducibility). The only
major difference from default settings is BIRD being pre-installed.

Unfortunately, using loop devices and mounting requires root privileges
(as far as I know). It should be possible to use some other way, but
this is the simplest I know at the moment.

gen_disks.sh

@@ -0,0 +1,30 @@
+#!/bin/sh
+
+# Run as root.
+
+set -eux
+
+mount="$(mktemp --directory)"
+trap "rmdir $mount" EXIT
+
+modprobe loop
+
+origdir="$(pwd)"
+
+for dir in output/*/; do
+	pushd $dir
+	cp "$origdir/dummydisk.img" "./disk.img"
+	loop="$(losetup --partscan --show --find ./disk.img)"
+	mount -t ext4 "${loop}p3" "$mount"
+	while read what where; do
+		cp "$what" "$mount$where"
+	done << END
+		bird.conf /etc/bird.conf
+		interfaces /etc/network/interfaces
+		hosts /etc/hosts
+		hostname /etc/hostname
+END
+	umount "$mount"
+	losetup -d "$loop"
+	popd
+done