build: Export configure variables to hypervisor build

build: Export configure variables to hypervisor build

Since the introduction of autoconf, builds with XSM enabled in .config
have been broken unless FLASK_ENABLE was explicitly set. Since the
setting in .config has apparently been deprecated in favor of an
autoconf --enable-xsm, add config/Xen.mk to export this to Xen. This
also makes --disable-debug and some paths to be pulled from the
configure process in the hypervisor build.

Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Committed-by: Keir Fraser <keir@xen.org>