This will fetch a zip containing various ROM images, and building a single
combined.rom file, which is used by Xep128 then. It's loaded as-is into
EP memory linearly, it should contain ROM images appended to each other.

Emulator also uses its own ROM (XEP). It's now copied by the emulator
after the combined.rom, so there is no need here for combined.rom to
contain it. It also helps to solve the problem that combined.rom is
not so much Xep128 version dependent anymore caused by the XEP ROM. That
is, you don't need to update file combined.rom ever (hopefully, if I
don't change the structure of this file), unless you want to use/try
new ROMs of course.