EmulatorPkg: Enable support for Secure Boot

SECURE_BOOT_ENABLE feature flag is introduced to enable Secure Boot.
The following gets enabled with this patch:
* Secure Boot Menu in "Device Manager" for enrolling keys
* Storage space for Authenticated Variables
* Authenticated execution of 3rd party images

Signed-off-by: Divneil Rai Wadhawan <divneil.r.wadhawan@intel.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>