Makefile: add a make "dist" target

make dist can be used to produce tarballs directly from the git
repository, which can be useful to automate the release process as well
as shipping custom releases.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
diff --git a/Makefile b/Makefile
index afc0ce0..86f5ab3 100644
--- a/Makefile
+++ b/Makefile
@@ -196,6 +196,11 @@
 
 fdtput:	$(FDTPUT_OBJS) $(LIBFDT_archive)
 
+dist:
+	git archive --format=tar --prefix=dtc-v$(dtc_version)/ HEAD \
+		> ../dtc-v$(dtc_version).tar
+	cat ../dtc-v$(dtc_version).tar | \
+		gzip -9 > ../dtc-v$(dtc_version).tgz
 
 #
 # Testsuite rules