New sample program key_ladder_demo

Demo of a key derivation ladder.

Sample run in key_ladder_demo.sh.
diff --git a/programs/Makefile b/programs/Makefile
index c65a10c..9e1a527 100644
--- a/programs/Makefile
+++ b/programs/Makefile
@@ -60,7 +60,7 @@
 	pkey/rsa_decrypt$(EXEXT)	pkey/rsa_encrypt$(EXEXT)	\
 	pkey/rsa_sign$(EXEXT)		pkey/rsa_verify$(EXEXT)		\
 	pkey/rsa_sign_pss$(EXEXT)	pkey/rsa_verify_pss$(EXEXT)	\
-	psa/psa_constant_names$(EXEXT)	\
+	psa/key_ladder_demo$(EXEXT)	psa/psa_constant_names$(EXEXT)	\
 	ssl/dtls_client$(EXEXT)		ssl/dtls_server$(EXEXT)		\
 	ssl/ssl_client1$(EXEXT)		ssl/ssl_client2$(EXEXT)		\
 	ssl/ssl_server$(EXEXT)		ssl/ssl_server2$(EXEXT)		\
@@ -196,6 +196,10 @@
 	echo "  CC    pkey/rsa_encrypt.c"
 	$(CC) $(LOCAL_CFLAGS) $(CFLAGS) pkey/rsa_encrypt.c    $(LOCAL_LDFLAGS) $(LDFLAGS) -o $@
 
+psa/key_ladder_demo$(EXEXT): psa/key_ladder_demo.c $(DEP)
+	echo "  CC    psa/key_ladder_demo.c"
+	$(CC) $(LOCAL_CFLAGS) $(CFLAGS) psa/key_ladder_demo.c    $(LOCAL_LDFLAGS) $(LDFLAGS) -o $@
+
 psa/psa_constant_names$(EXEXT): psa/psa_constant_names.c $(DEP)
 	echo "  CC    psa/psa_constant_names.c"
 	$(CC) $(LOCAL_CFLAGS) $(CFLAGS) psa/psa_constant_names.c    $(LOCAL_LDFLAGS) $(LDFLAGS) -o $@