[Coco] Proof-of-concept to make 16KB HDB-DOS ROMs seamlessly

[Juan Castro]

I replicated the "two byte ASCII signature" scheme to allow for playing
with those unused top 8KB of ROM space. The example just puts an
exclamation point at the top right of the screen, but you can do anything.
Tell me what you think.

I made it so, if the top 8KB don't exist, it still works as before. I had
to condense a couple of messages. The makefile builds the normal 8K ROMs as
before, and also the 16K roms (prefix el-).

(This is a patch for the current source in Toolshed's Mercurial.)

https://dl.dropboxusercontent.com/u/15643089/hdbdos-16krom.patch

Juan