scripts: add "prune-kernel" script to clean up old kernel images

scripts: add "prune-kernel" script to clean up old kernel images

Long ago, Dave Jones complained about CONFIG_LOCALVERSION_AUTO:
 "I don't use the auto config, because I end up filling up /boot unless
  I go through and clean them out by hand every time I install a new one
  (which I do probably a dozen or so times a day).  Is there some easy
  way to prune old builds I'm missing?"

To which Bruce replied:
 "I run this by hand every now and then.  I'm probably doing it all wrong"

And if he is running it wrong, then so am I - because I've been using
this script ever since.  It is true that CONFIG_LOCALVERSION_AUTO easily
ends up filling your /boot partition if you don't clean up old versions
regularly, and this script helps make that easier.

Checked with Bruce to see that it's fine to add this to the kernel
scripts.  Maybe people will come up with enhancements, but more
importantly, this way I won't misplace this script whenever I install a
new machine and start doing custom kernels for it.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>