kbuild: make kernelrelease

This patch makes it easy to programmatically get at the kernel
makefile's idea of the kernel version from external scripts and
makefiles with something like V=`make kernelrelease`.

Alternatives include parsing Makefile (errorprone and broken by things
like localversion) and running the C preprocessor on version.h (which
requires a) building version.h somewhere and b) is really ugly).

From: Matt Mackall <mpm@selenic.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

Makefile

@@ -1210,6 +1210,9 @@ checkstack:
 	$(OBJDUMP) -d vmlinux $$(find . -name '*.ko') | \
 	$(PERL) $(src)/scripts/checkstack.pl $(ARCH)
 
+kernelrelease:
+	@echo $(KERNELRELEASE)
+
 # FIXME Should go into a make.lib or something 
 # ===========================================================================