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/configs/blacklist/BLACKLIST-latest

@@ -1,7 +1,5 @@
 # TEMPORARILY DISABLED
 send_signal		# flaky
 test_lsm		# semi-working
-sk_assign		# needs better setup in Travis CI
-sk_lookup
 core_reloc		# temporary test breakage
 bpf_verif_scale		# clang regression