[util] Add mergerom.pl

[util] Add mergerom.pl

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.