convert FLASK_ENABLE to Kconfig

convert FLASK_ENABLE to Kconfig

Converts the Config.mk option of FLASK_ENABLE into a Kconfig option for
the hypervisor called CONFIG_FLASK. This commit knowingly breaks the
dependent relationship on XSM_ENABLE which is addressed when XSM_ENABLE
is converted to Kconfig.

Signed-off-by: Doug Goldstein <cardoe@cardoe.com>
Acked-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>