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

This brings us in to line with Linux definitions, and also simplifies
adding x86_64 support since both platforms have 2-byte shorts, 4-byte
ints and 8-byte long longs.

Apparently some BIOSes will place option ROMs on 512-byte boundaries.
While this is against specification, it doesn't actually hurt
anything, so we may as well increase our scan granularity to 512
bytes.

Contributed by Luca <lucarx76@gmail.com>

undi_rom rather than struct pxe_driver.  (This is part of the naming
consistency fixes).