If the libfdt-packe is missing or is too old, QEMU reports the
following error:

ERROR: DTC (libfdt) version >= 1.4.2 not present. Your options:
         (1) Preferred: Install the DTC (libfdt) devel package
         (2) Fetch the DTC submodule, using:
             git submodule update --init dtc

It looks like the version of libfdt-dev that is currently deployed on
travis-ci.org is too old. So, let's do (2).

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>
Reviewed-by: Igor Opaniuk <igor.opaniuk@linaro.org>

All supported projects in build.git besides DRA7xx will be fully built
when Travis is enabled. This configuration will try to download and
cache the toolchains and ccache (to be verified).

Signed-off-by: Joakim Bech <joakim.bech@linaro.org>
Reviewed-by : Jerome Forissier <jerome.forissier@linaro.org>