Add support for manipulating the jump instruction that forms the
option ROM initialisation entry point, so that mergerom.pl can treat
it just like other entry points.

Add support for merging the initialisation entry point (and IBM BOFM
table) to mergerom.pl; this is another slightly icky but unfortunately
necessary GPL vs. NDA workaround.  When mergerom.pl replaces an entry
point in the original ROM, it now fills in the corresponding entry
point in the merged ROM with the original value; this allows (for
example) a merged initialisation entry point to do some processing and
then jump back to the original entry point.

This utility is required as a workaround for legal restrictions on
including GPL and non-GPL code within the same expansion ROM image.

While this is not encouraged, we are prepared to accept that
concatenation of ROM images and updating of the ROM header data
structures can be classed as "mere aggregation" within the terms of
the GPL.

If in any doubt, assume that you cannot include GPL and non-GPL code
within the same expansion ROM image.  Contact the Etherboot team for
clarification on your specific circumstances.