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: 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>