Add support for lockdown

[ricardon]

We need to make sure that Linux and firmware handles correctly Secure Boot and related EFI variables. lockdown is a tool to populate the relevant UEFI Secure Boot variables. This can help to prevent tests related to UEFI Secure Boot from being skipped and potentially hiding issues.

The tool can be found here:https://github.com/vathpela/lockdown

[km13park]

This issue is related with "Build OVMF with support for secure boot #38". Instead of using lockdown, we can use EnrollDefaultKeys.efi to enroll Microsoft signed public keys and run the test suites with secureboot mode. Please refer to issue #38 for the detailed step.