[fnrec] Add function recorder for debugging

[fnrec] Add function recorder for debugging

The function recorder is a crash and hang debugging tool.  It logs each
function call into a memory buffer while gPXE runs.  After the machine
is reset, and if the contents of memory have not been overwritten, gPXE
will detect the memory buffer and print out its contents.

This allows developers to see a trace of the last functions called
before a crash or hang.  The util/fnrec.sh script can be used to convert
the function addresses back into symbol names.

To build with fnrec:

    make FNREC=1

Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>
Signed-off-by: Marty Connor <mdc@etherboot.org>