People relied on the build generation number to find out if they actually
booted the right kernel, only incrementing it on config changes isn't
enough for them. So this patch goes back to the old behavior, only done
right this time:

We now don't increment the generation number on every invocation of make, 
but on every link of vmlinux. At the same time, this patch fixes the
Makefile to only relink vmlinux if any of its prequisite object files
(or the command line) changed. Incrementing the version number
happens *after* we decided to relink vmlinux.