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>