Merge tag 'efi-2022-04-rc2-4' of https://source.denx.de/u-boot/custodians/u-boot-efi

Pull request for efi-2022-04-rc2-4

Documentation:

* mkeficapsule man-page

UEFI changes:

* add support for signing images to mkeficapsule
* add support for user define capsule GUID
* adjust unit tests for capsules
* fix UEFI image signature validation in case of multiple signatures