ci: Add Travis CI infrastructure

ci: Add Travis CI infrastructure

Based on Roger Meier's proposal, this adds support for testing Jailhouse
builds on Travis CI (travis-ci.org). The major differences to Roger's
approach are:
 - Linux kernels are pre-built and pushed as archive to a webserver
 - all target variants (x86, Banana Pi, Versatile Express) are built in
   a single run to limit archive downloads
 - required kernel and Jailhouse configs become part of our tree

The kernel archive can be generated via ci/gen-kernel-build.sh in an
environment comparable to the Travis CI VMs. See ci/README.md for more
information.

CC: Roger Meier <r.meier@siemens.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>