grub: Enable secure boot

Leo Yan requested to merge leo.yan/meta-ledge-secure:enable_grub_secure into main

This patch enables secure boot for GRUB.

GRUB uses PGP for key management, we need to import PGP private key and then use the private key to sign images. For this purposes, this patch set creates the pgp_sign.bbclass file to keep the common code for importing key and signing image.

Then this patch enables signing in separate recipes for GRUB config file, Linux kernel and ramdisk images.

Signed-off-by: Leo Yan

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