tools: make FLASK utils build unconditional

tools: make FLASK utils build unconditional

The flask utilities only have dependencies on libxc so there's no
downside to always building it. Distros and projects based on Xen can
put these in a different package to not install them for all users.
Prior to this change FLASK_ENABLE needed to be set at the top level to
build the utilities and the tools/configure script would build the FLASK
policy by default, but only if the utilities were built.

This change makes item 3 from
http://lists.xenproject.org/archives/html/xen-devel/2016-01/msg01796.html
a happen by default.

CC: Ian Jackson <ian.jackson@eu.citrix.com>
CC: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
CC: Ian Campbell <ian.campbell@citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: Doug Goldstein <cardoe@cardoe.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>