travis.yml: Update Travis to use Bionic and Focal instead of Xenial

travis.yml: Update Travis to use Bionic and Focal instead of Xenial

According to our support policy, we do not support Xenial anymore.
Time to switch the bigger parts of the builds to Focal instead.
Some few jobs have to be updated to Bionic instead, since they are
currently still failing on Focal otherwise. Also "--disable-pie" is
causing linker problems with newer versions of Ubuntu ... so remove
that switch from the jobs now (we still test it in a gitlab CI job,
so we don't lose much test coverage here).

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Cleber Rosa <crosa@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Message-Id: <20200918103430.297167-6-thuth@redhat.com>
Message-Id: <20200925154027.12672-7-alex.bennee@linaro.org>