Hello!

I have been putting much time and effort to debug and understand the bug, that lead to this error message:

Invalid content for hash comparison
failed to verify file buffer (err: 0x800000000000001a)
error: failed to verify initrd /initrd.

Press any key to continue...

The error is EFI_SECURITY_VIOLATION from the EFI Error codes. Funnily, all the other files (grub.cfg, bzImage) could get verified okay and also my "simple example" just using meta-secure-core worked.
Finally, I found the cause of the error: grub uncompressed the cpio.gz initrd and compared the extracted contents with the signature of the compressed initrd. The difference was, that I am using squash4 module of grub since I am using squashfs for the target system.

The workaround is to set the INITRAMFS_FSTYPES to for example cpio.

I just wanted to leave this here as a caveat.