kbuild: KERNELRELEASE is only re-defined when buiding the kernel

kbuild: KERNELRELEASE is only re-defined when buiding the kernel

To avoid running setlocalversion as root no longer (re-)define
KERNELRELEASE for each run. With this patch KERNELRELEASE is
only re-read when we do an actual kernel build.
Rationale behind this is "do as little as possible" when executing
make install - as root!

A new file named .kernelrelease is strored in the root of the kernel
tree containing the actual version string.
So when we use do a kernel build the .kernelrelease file will be updated.
But in all other situations it is left as-is.

To make it more visible the kernel now prints out the version being build.
Sample:
Building kernel 2.6.15-g63b794bf-dirty
...
...

The patch also un-exports VERSION PATCHLEVEL SUBLEVEL EXTRAVERSION LOCALVERSION
since all users of these are anyway broken - and none is left in the
tree.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>