SPDX specification change: addition of NONE and NOASSERTION to license expressions

[pmonks]

As of SPDX 3.1 (I think...), license expressions will support NONE and NOASSERTION as pseudo-identifiers.

This may be implemented by adding those two values as actual identifiers in the license list (that's one proposal), or perhaps as special cased values in the simple-expression rule in the license expression grammar. If the former solution is implemented, no change should be necessary to the parser (it already reads the SPDX license list to determine the definition of the license-id rule), but if the latter approach is implemented then the parser here will need to be updated to match.

[zvr]

FYI, it will be in SPDX 3.0.