docker: Add flex and bison to centos6 image

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: NThomas Huth <thuth@redhat.com>
Signed-off-by: NFam Zheng <famz@redhat.com>
Message-Id: <20170524005206.31916-1-famz@redhat.com>
Reviewed-by: NAlex Bennée <alex.bennee@linaro.org>
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: NFam Zheng <famz@redhat.com>

tests/docker/dockerfiles/centos6.docker

 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