GRUB2: avoid loading unsigned kernels when GRUB is booted directly under secureboot without shim

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grub2: Fail kernel validation without shim protocol

GRUB2 fails to validate kernel signature when booted directly without shim, allowing secure boot to be bypassed. This only affects systems where the kernel signing certificate has been imported directly into the secure boot database and the GRUB image is booted directly without the use of shim...