build-target-bins.sh: include tb_fw_config dtb in fip image

Commit 3cb84a5 ("plat/arm: Rentroduce tb_fw_config device tree") in the
Arm Trusted Firmware project [1] introduced changes such that the
fw_config device tree will maintain the information shared across all
the BLx images while the tb_fw_config device tree will hold the BL2
configuration nodes. Update build-target-bins.sh file to include both
fw_config and tb_fw_config device trees during the fip image creation
stage.

[1]: https://git.trustedfirmware.org/TF-A/trusted-firmware-a.git/commit/?id=3cb84a54258e148ba0240fc5fbe4ef828339c13d



Change-Id: Ica4aede67782c42686a2971d50601eca86609d52
Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>