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_tweak.sh

@@ -0,0 +1,61 @@
+#!/bin/bash
+# This script prepares a mounted root filesystem for testing libbpf in a virtual
+# machine.
+set -e -u -x -o pipefail
+root=$1
+shift
+
+chroot "${root}" /bin/busybox --install
+
+cat > "$root/etc/inittab" << "EOF"
+::sysinit:/etc/init.d/rcS
+::ctrlaltdel:/sbin/reboot
+::shutdown:/sbin/swapoff -a
+::shutdown:/bin/umount -a -r
+::restart:/sbin/init
+EOF
+chmod 644 "$root/etc/inittab"
+
+mkdir -m 755 -p "$root/etc/init.d" "$root/etc/rcS.d"
+cat > "$root/etc/rcS.d/S10-mount" << "EOF"
+#!/bin/sh
+
+set -eux
+
+/bin/mount proc /proc -t proc
+
+# Mount devtmpfs if not mounted
+if [[ -z $(/bin/mount -l -t devtmpfs) ]]; then
+	/bin/mount devtmpfs /dev -t devtmpfs
+fi
+
+/bin/mount sysfs /sys -t sysfs
+/bin/mount bpffs /sys/fs/bpf -t bpf
+/bin/mount debugfs /sys/kernel/debug -t debugfs
+
+echo 'Listing currently mounted file systems'
+/bin/mount
+EOF
+chmod 755 "$root/etc/rcS.d/S10-mount"
+
+cat > "$root/etc/rcS.d/S40-network" << "EOF"
+#!/bin/sh
+
+set -eux
+
+ip link set lo up
+EOF
+chmod 755 "$root/etc/rcS.d/S40-network"
+
+cat > "$root/etc/init.d/rcS" << "EOF"
+#!/bin/sh
+
+set -eux
+
+for path in /etc/rcS.d/S*; do
+	[ -x "$path" ] && "$path"
+done
+EOF
+chmod 755 "$root/etc/init.d/rcS"
+
+chmod 755 "$root"