.travis.yml: initial build matrix

This adds a TravisCI build matrix to test all the various build
combinations of architecture and with/without out-of-tree builds. It
currently fails because the Trusty compilers have some issues with
various format strings and unsigned/signed comparisions but these can
be fixed up later.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>