Simplify digest-to-efi-sign-list

[chwallen]

Hey, thanks for this repo! Really helped me with trusting the VBIOS for my GPU and constantly keeping the Windows boot manager signature in sync with my db.

I did make a few simplifications to the digest-to-efi-sig-list:

• use the existing EFI_SHA256_HASH (defined as typedef UINT8 EFI_SHA256_HASH[32];) instead of hacking with a UINT8[1]
• merge the two structs into one, which allows setting the fields in the initializer instead of hacking with unsigned char[]
• write the parsed digest directly into the struct, thanks to the point above