vmtest: update root fs, whitelist sk_{assign|lookup} test

1. Update mkrootfs.sh building root fs
- Remove /etc/fstab from root fs and mount each fs type separately in
S10-mount script.
- devtmpfs can be already mounted prior to S10-mount execution so make
it opt-out. This addresses [0].
- set -eux for scripts
2. Add iproute2 to root fs and whitelist sk_assign test. Addresses
[1][2]. Update INDEX file with 2020-09-27 version.

[0] https://github.com/libbpf/libbpf/pull/145#issuecomment-609673493
[1] https://github.com/libbpf/libbpf/pull/144
[2] https://github.com/libbpf/libbpf/pull/145

travis-ci/vmtest/run.sh

@@ -411,7 +411,7 @@ if [[ ! -z SETUPCMD ]]; then
 	if [[ -v BUILDDIR ]]; then kernel='latest'; fi
 	setup_envvars="export KERNEL=${kernel}"
 	setup_script=$(printf "#!/bin/sh
-set -e
+set -eux
 
 echo 'Running setup commands'
 %s