Add support for deleting specific keys
Make sure the menu shows when the callback fails

Since Pause() doesn't clear the key from the input queue, the next
ReadKeyStroke reads the queued key instead of the new one. If the
user presses "Enter", MokManager exits directly without showing
the menu again.
Verify efi images with PK and KEK

The user may sign the EFI image with PK or KEK if the machine was
shipped with setup mode.