target-bin: fix incorrect dynamic config parameters passed to fiptool

The patch a55a9a52 ("target-bin: allow tb-fw-config and hw-config to be
part of uefi fip") incorrectly assumes the availability of the hw_config
dtb file if tb_fw_config dtb file is present. Fix this by checking for
available hardware config dtb files and then generate corresponding
parameters for the fip tool.

Change-Id: I58bf325bcdac2c3ab99ba2b6c17f9e114136d885
Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>