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>

Config.mk

@@ -214,7 +214,6 @@ EMBEDDED_EXTRA_CFLAGS += -fno-exceptions
 
 # Enable XSM security module (by default, Flask).
 XSM_ENABLE ?= n
-FLASK_ENABLE ?= $(XSM_ENABLE)
 
 XEN_EXTFILES_URL ?= http://xenbits.xen.org/xen-extfiles
 # All the files at that location were downloaded from elsewhere on