qemu arm64/amd64: enable secure boot

Luca Boccassirequested to merge
bluca/u-boot:qemu_secure_boot into debian/latest

Secure boot is now supported upstream in EFI mode. It is disabled by default, and can be enabled by loading keys from the console:

https://u-boot.readthedocs.io/en/latest/develop/uefi/uefi.html#configuring-uefi-secure-boot

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