vmtest: add debootstrap-based mkrootfs script

The existing mkrootfs.sh is based on Arch Linux, which supports only
x86_64.

Add mkrootfs_debian.sh: a debootstrap-based script. Debian was chosen,
because it supports more architectures than other mainstream distros.
Move init setup to mkrootfs_tweak.sh, rename the existing Arch script
to mkrootfs_arch.sh.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>

travis-ci/vmtest/mkrootfs_debian.sh

@@ -0,0 +1,40 @@
+#!/bin/bash
+# This script builds a Debian root filesystem image for testing libbpf in a
+# virtual machine. Requires debootstrap >= 1.0.95 and zstd.
+
+set -e -u -x -o pipefail
+
+# Check whether we are root now in order to avoid confusing errors later.
+if [ "$(id -u)" != 0 ]; then
+	echo "$0 must run as root" >&2
+	exit 1
+fi
+
+# Create a working directory and schedule its deletion.
+root=$(mktemp -d -p "$PWD")
+trap 'rm -r "$root"' EXIT
+
+# Install packages.
+packages=binutils,busybox,elfutils,iproute2,libcap2,libelf1,strace,zlib1g
+debootstrap --include="$packages" --variant=minbase bullseye "$root"
+
+# Remove the init scripts (tests use their own). Also remove various
+# unnecessary files in order to save space.
+rm -rf \
+	"$root"/etc/rcS.d \
+	"$root"/usr/share/{doc,info,locale,man,zoneinfo} \
+	"$root"/var/cache/apt/archives/* \
+	"$root"/var/lib/apt/lists/*
+
+# Save some more space by removing coreutils - the tests use busybox. Before
+# doing that, delete the buggy postrm script, which uses the rm command.
+rm -f "$root/var/lib/dpkg/info/coreutils.postrm"
+chroot "$root" dpkg --remove --force-remove-essential coreutils
+
+# Apply common tweaks.
+"$(dirname "$0")"/mkrootfs_tweak.sh "$root"
+
+# Save the result.
+name="libbpf-vmtest-rootfs-$(date +%Y.%m.%d).tar.zst"
+rm -f "$name"
+tar -C "$root" -c . | zstd -T0 -19 -o "$name"