flask: build policy in different locations

flask: build policy in different locations

The flask policy can be build twice -- one for hypervisor and one for
tools.

Before this patch, everything is built inside tools/flask/policy
directory.  It is possible to have a race to write to the same output
file when running parallel builds.

Prepend output file names with FLASK_BUILD_DIR. Hypervisor and tools
build will set that variable to different directories, so that we can
be safe from races.

Adjust other bits of the build system as needed.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>