Clarify some TODO items.

- save/restore of the CPU state is not implemented (see machine.c).

- Improve cpu state handling:

  Step 1) Collect all the TC state in a single struct, so we need only

          a single global pointer per TC.

          a single global pointer for the active TC.

  Step 2) Use only a single TC context as working context, and copy the

          contexts on TC switch. Likewise for FPU contexts.

"Generic" 4Kc system emulation

------------------------------

- Doesn't correspond to any real hardware.

- Doesn't correspond to any real hardware. Should be removed some day,

  U-Boot is the last remaining user.

PICA 61 system emulation

------------------------

MALTA system emulation

----------------------

- We fake firmware support instead of doing the real thing

- Real firmware falls over when trying to init RAM, presumably due

  to lacking system controller emulation.

- Real firmware (YAMON) falls over when trying to init RAM, presumably

  due to lacking system controller emulation.

- Bonito system controller not implemented

- MSC1 system controller not implemented