Fix start of compatible image

The compatible image has to start one sector later, so that the filesystem
signature is at the beginning of the file. That way, the emulator will
prepend enough empty sectors so that the boot loader will start loading
from the correct position.
Fix INC(var,const) for large constants

Thanks Jörg for the fix on the mailing list.

https://lists.inf.ethz.ch/pipermail/oberon/2014/007596.html