images : Add a file /etc/version-image.

This file contains a timestamp, the image recipe name, and the value
of the variable RELEASE_NAME. Set RELEASE_NAME in local.con.

Signed-off-by: Philip Balister <philip@balister.org>
gnuradio : Update to version 3.7.5.

Signed-off-by: Philip Balister <philip@balister.org>