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.

Probe mechanisms now use standard EISA and MCA bus methods, with 3c509.c
defining a custom ISA bus to handle the 3c509 contention resolution logic.

MCA support now provided by mca.c