tools: Commit flex (2.6.4) & bison (3.3.2) output from Debian buster

tools: Commit flex (2.6.4) & bison (3.3.2) output from Debian buster

These files are in tree so that people can build (including from git)
without needing less-than-a-decade-old flex and bison.

We should update them periodically.  Debian buster has been Debian
stable for a while.  Our CI is running buster.

There should be no significant functional change; it's possible that
there are bugfixes but I have not reviewed the changes.  I *have*
checked that the flex I am using has the fix for CVE-2016-6354.

CC: Paul Durrant <paul@xen.org>
CC: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
Release-acked-by: Paul Durrant <paul@xen.org>