armv7R: K3: am654: Add support for generating build targets

Update Makefiles to generate:
- tiboot3.bin: Image format that can be processed by ROM.

Below is the tiboot3.bin image format that is required by ROM:

		 _______________________
		|	 X509		|
		|     Certificate	|
		| ____________________	|
		| |		      |	|
		| | u-boot-spl.bin    |	|
		| |		      |	|
		| |___________________|	|
		|_______________________|

Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Andreas Dannenberg <dannenberg@ti.com>