Create Makefile target/helper to build Heads payload seperately from coreboot

[tlaurion]

Vaultboot did it a while ago, and it's actually a good idea to do so, even more when the goal of having some generic linux requirements and target/* requirements being streamlined.

For reference, vaulboot did this under all in one commit hardenedvault/vaultboot@09e72b9, look for Makefile payload target https://github.com/hardenedvault/vaultboot/blame/master/Makefile