firmware: Add separate dummy payload for FW_PAYLOAD

Currently, the default payload for FW_PAYLOAD is embedded
fw_payload.S itself. This means people have to hack fw_payload.S
if they want to have some temporary S-mode test code.

This patch adds a separate dummy payload for FW_PAYLOAD which
can be easily hacked for some S-mode testing.

Signed-off-by: Anup Patel <anup.patel@wdc.com>

firmware/payloads/objects.mk

@@ -0,0 +1,19 @@
+#
+# Copyright (c) 2018 Western Digital Corporation or its affiliates.
+#
+# Authors:
+#   Anup Patel <anup.patel@wdc.com>
+#
+# SPDX-License-Identifier: BSD-2-Clause
+#
+
+firmware-bins-$(FW_PAYLOAD) += payloads/dummy.bin
+
+dummy-y += dummy_head.o
+dummy-y += dummy_main.o
+
+%/dummy.o: $(foreach obj,$(dummy-y),%/$(obj))
+	$(call merge_objs,$@,$^)
+
+%/dummy.dep: $(foreach dep,$(dummy-y:.o=.dep),%/$(dep))
+	$(call merge_deps,$@,$^)