Use git describe to set KERNELVERSION

Change-Id: Id579b19fc38c7ca2b98ad1e87aaec71c070a9178
Signed-off-by: Peter Stuge <peter@stuge.se>
Reviewed-on: http://review.coreboot.org/163
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>
diff --git a/Makefile.inc b/Makefile.inc
index 89467db..6c51a84 100644
--- a/Makefile.inc
+++ b/Makefile.inc
@@ -19,8 +19,7 @@
 
 #######################################################################
 # misleadingly named, this is the coreboot version
-REV=-r$(shell if [ -d $(top)/.git -a -f "`which git`" ]; then git --git-dir=/$(top)/.git show -s --pretty=format:%h; fi)
-export KERNELVERSION      := 4.0$(REV)
+export KERNELVERSION := $(shell if [ -d "$(top)/.git" -a -f "`which git`" ]; then git describe; else echo unknown; fi)
 
 #######################################################################
 # Basic component discovery