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

list.

Added FEATURE() macros to most relevant (non-driver) files.

UNDI stack.

typical build will now include 880 bytes of PCI support code, compared to
2327 bytes in Etherboot 5.4.

(There is a slight cost of around 5 extra bytes per access to a
non-constant config space address; this should be an overall win.
Driver-specific accesses will usually be to constant addresses, for
which there is no additional cost.)

Yes, you really can do it in 65 bytes.