Rename efi_nvram build tag and say something about it in README

Comment by @nisse in !13 (merged):

nit: I would have preferred something looking less neutral, e.g., clobber_efi_nvram, and maybe something making clear that it's about tests; it's not a build tag to enable/disable nvram access in stprov itself. Also deserves some documentation, in README.md if there's no better place.

Please do a followup mr, or file an issue.