build: figure out git revision and include as part of version string

Signed-off-by: Vegard Nossum <vegard.nossum@gmail.com>

scripts/localversion.sh

@@ -0,0 +1,21 @@
+#! /bin/bash
+
+#
+# Use git to figure out which version we are using.
+#
+# Adapted from a script written by Rene Scharfe <rene.scharfe@lsrfire.ath.cx>,
+# found in the Linux Kernel.
+#
+
+set -u
+
+if head=`git rev-parse --verify --short HEAD 2>/dev/null`; then
+	printf '%s%s' -g $head
+
+	git update-index --refresh --unmerged > /dev/null
+	if git diff-index --name-only HEAD | read dummy; then
+		printf '%s' -dirty
+	fi
+
+	exit
+fi