ti: omap-common: Add commands for generating secure SPL images

ti: omap-common: Add commands for generating secure SPL images

Adds a centralized config_secure.mk in omap-common for
OMAP-style TI secure devices to use for boot image generation

Depending on the boot media, different images are needed for
secure devices. These commands generates u-boot*_HS_* files that
need to be used to boot secure devices.

Please refer to README.ti-secure for more information.

Signed-off-by: Daniel Allred <d-allred@ti.com>
Signed-off-by: Madan Srinivas <madans@ti.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>
Tested-by: Andreas Dannenberg <dannenberg@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>