Add null attributes delete variable

As par EDK-2 and EDK-2 test code, setVariable() with 0
attributes means a delete variable request. Currently,
smm gatway doesn't handle this scenario. This commit adds
that support.

Signed-off-by: Emekcan Aras <emekcan.aras@arm.com>
Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Change-Id: Id9801838dc12774393726a3d2b9c9f88ca82d060
1 file changed