At least one Dell system calls the UNDI loader entry point with the
BIOS console disabled.  The serial console is active only after a call
to initialise(), so move the debug message in undi_loader() so that it
can be displayed via the serial console.

__from_data16 and __from_text16 now take a pointer to a
.data16/.text16 variable, and return the real-mode offset within the
appropriate segment.  This matches the use case for every occurrence
of these macros, and prevents potential future bugs such as that fixed
in commit d51d80f.  (The bug arose essentially because "&pointer" is
still syntactically valid.)

defined in vsprintf.h.  (This may change, since vsprintf.h is a
non-standard name, but for now it's the one to use.)

There should be no need to include vsprintf.h just for DBG() statements,
since include/compiler.h forces it in for a debug build anyway.

PXE code now compiles without errors (though it won't actually work).