Output flatc and reflection.fbs to known location

regenerate_headers.sh & pytest tests expect to find flatc and (in the
latter case) reflection.fbs in a particular location in the source tree.

Signed-off-by: Michael Platings <michael.platings@arm.com>
Change-Id: Ide69e5cf75a7a33cdc022776e9e3fa3d73662e9a