Currently there are warnings about flex and bison being missing when
building in the centos6 image:

    make[1]: flex: Command not found
             BISON dtc-parser.tab.c
    make[1]: bison: Command not found

Add them.

Reported-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
Message-Id: <20170524005206.31916-1-famz@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Fam Zheng <famz@redhat.com>
---
 tests/docker/dockerfiles/centos6.docker | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tests/docker/dockerfiles/centos6.docker b/tests/docker/dockerfiles/centos6.docker
index 34e0d3b..17a4d24 100644
--- a/tests/docker/dockerfiles/centos6.docker
+++ b/tests/docker/dockerfiles/centos6.docker
@@ -1,7 +1,7 @@
 FROM centos:6
 RUN yum install -y epel-release
 ENV PACKAGES libfdt-devel ccache \
-    tar git make gcc g++ \
+    tar git make gcc g++ flex bison \
     zlib-devel glib2-devel SDL-devel pixman-devel \
     epel-release
 RUN yum install -y $PACKAGES
-- 
2.9.4