[contrib] Add patch to enable writable ROM images in bochs
Some of the gPXE romprefix.S code attempts to write to the ROM image,
as allowed by the PCI spec. We do gracefully handle the case of
read-only ROM images (for ISA ROMs and for normal bochs/qemu
operation), but it can be handy to be able to use bochs to debug the
code paths that depend on writable ROM images.