[MiNT] FreeMiNT MMU usage define

[Vincent Rivière <vincent.riviere@freesbee.fr>]

Hello.

In order to clean-up things I would like to totally disable the FreeMiNT MMU 
usage in mintv4e.prg (until someone writes support for the ColdFire MMU). 
Currently FreeMiNT thinks there is MMU support while there is not.

Currently, the FreeMiNT MMU usage is conditioned by a #ifdef M68000 so of 
course this is not satisfying for ColdFire.

I would like to use a new define to enable FreeMiNT's MMU usage. For 
example, USE_MMU would be fine. An alternative would be NO_MMU (like 
NO_RAMFS, NO_FAKE_SUPER...), but I don't like negative defines.

If everyone agrees, I'm going to add -DUSE_MMU in the KERNELDEFS for all the 
kernels except 68000 and ColdFire, then test that define in the sources.

--
Vincent Rivière