Add FILE_LICENCE declarations to almost all files that make up the
various standard builds of gPXE.

ISA 3c509 is currently non-functional, although the EISA (3c509-eisa) and
MCA (3c529) variants should build OK.

None of this code is yet tested.

initial device, and will also allow for e.g. a device menu to be presented
to the user.

IDs in a human-readable format.