Option::ROM was assuming that ROM images using a short jump
instruction for the init entry point would have a zero byte at offset
5; this is not necessarily true.

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.

The Option::ROM module provides an easy way to read and edit fields
within option ROM headers.