Add bitbake-dumpsig to make it more obvious how to dump a signatures file

[YOCTO #2057]

(Bitbake rev: c5fdc6c02fe4712d3114caccfd759a626bd9677b)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/bin/bitbake-dumpsig

@@ -0,0 +1,9 @@
+#!/usr/bin/env python
+import os
+import sys
+import warnings
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.dirname(sys.argv[0])), 'lib'))
+
+import bb.siggen
+
+bb.siggen.dump_sigfile(sys.argv[1])