docker: Fix travis script unable to find source dir

The script generated from QEMU_SRC/.travis.yml uses BUILD_DIR and
SRC_DIR path relative to the current dir, unless these variables
are exported in environment.

Since commit 05790da BUILD_DIR is exported in the runner script,
although SRC_DIR is not, so that make docker-travis fails becase
the reference to source dir is wrong. So let's unset both BUILD_DIR
and SRC_DIR before calling the script, given it is executed from
the source dir already (as in Travis).

Signed-off-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Message-Id: <20190320221207.11366-3-wainersm@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

tests/docker/travis

@@ -18,4 +18,5 @@ cmdfile=/tmp/travis_cmd_list.sh
 $QEMU_SRC/tests/docker/travis.py $QEMU_SRC/.travis.yml > $cmdfile
 chmod +x $cmdfile
 cd "$QEMU_SRC"
+unset BUILD_DIR SRC_DIR
 $cmdfile