testing/coreboot-tools: add cbmem subpkg

First PR here, I've been using this package for a little while and it's working. Please tell me if I've made any mistakes in this contribution!

One question: Why not add all of the tools as subpackages? I know, at least for me, that needing to have a coreboot tree around to enter the util directory whenever I need a tool like intelvbttool is pretty annoying.