Since we'll have to always do module postprocessing shortly, we can as well
get rid of the special cased init/vermagic.o which needed to be compiled
before descending, and instead include the current version magic string
during post processing. For that purpose, the generation of the string is
moved from init/vermagic.c to include/linux/vermagic.h.

People who externally maintain modules will also be happy about that.