vmtest: add configs for bpf kernel selftests

vmtest is run as a TravisCI job in order to test libbpf backward compatibility
with the older kernels

Add config files required to build and run bpf kernel selftests in vmtest:
- latest.config: latest kernel config
- INDEX: links to binaries (kernels, disk image) to download
- blacklist/BLACKLIST-${kernel}: blacklisted bpf program tests for ${kernel}

travis-ci/vmtest/configs/blacklist/BLACKLIST-5.5.0-rc6

@@ -0,0 +1,18 @@
+mmap
+dctcp
+cubic
+bpf_tcp_ca
+bpf_verif_scale
+cgroup_attach
+pinning
+send_signal_tracepoint_thread
+test_syncookie
+select_reuseport
+send_signal
+sockopt_inherit
+strobemeta_nounroll2
+stacktrace_build_id
+tp_attach_query
+tcp_rtt
+task_fd_query_tp
+stacktrace_map