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>

Makefile

@@ -196,6 +196,11 @@ fdtget:	$(FDTGET_OBJS) $(LIBFDT_archive)
 
 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