Remove more useless stuff from the cpc target.

These files are not referenced in contiki.lnk, so they weren't getting
linked in the binary, But they added a lot to the build time...
Fix the remaining icons to be const...
WIP support for drawing bitmaps

* The bitmap is relocated by the loader,
* The bitmap is drawn as 9 custom characters, so we setup a custom
character area (of 1 char, to save memory), and define and draw custom
characters.
* Not enabled yet as this works with the dynamically loaded icons but
not the ones on the desktop.
Remove configedit from the system.

It is also available as an external program...
Enable the exit menu, which will reset the machine.

A debatable decision, ctrl+shift+esc works anyway, and the menu with
confirmation dialog needs about 200 bytes.