The MIT and ISC licenses are legally equivalent to the bsd2 license,
but with slightly different verbiage.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Michael Brown <mcb30@etherboot.org>

It is now possible to run e.g.

  make bin/rtl8139.dsk.licence

in order to see a licensing assessment for any given gPXE build.  The
assessment will either produce a single overall licence for the build
(based on combining all the licences used within the source files for
that build), or will exit with an error stating why a licence
assessment is not possible (for example, if there are files involved
that do not yet contain an explicit FILE_LICENCE() declaration).