Better documentation of requirements and EBBR versions

[vstehle]

We could improve the following aspects of requirements and versions:

• Better unique identification of each requirement
• Better documentation of Which version of EBBR introduced each requirement

This would ease linking an EFI_CONFORMANCE_PROFILE EBBR GUID to a set of requirements.

This has been reported by @xypron on the ML.

E-mail extract:

I guess a compliant software present all four of the GUIDs so that a client
looking for a certain compliance level will hit a match.

Currently U-Boot only presents the EFI_CONFORMANCE_PROFILE_EBBR_2_1_GUID. We
should evaluate what is missing with respect to the other levels.

What I am missing in the EBBR spec is a table with the requirements indicating
in which version each requirement was added.

Testing would also become easier if each requirement were numbered.

This is somewhat related with #118, too.