spl: build: ti: add support for secure boot images

spl: build: ti: add support for secure boot images

Updates the SPL build so that when CONFIG_TI_SECURE_DEVICE
is in use (which it should be when building for secure parts),
the TI secure development package is used to create a valid
secure boot image. The u-boot SPL build processes is NOT aware
of the details of creating the boot image - all of that information
is encapsulated in the TI secure development package, which is
available from TI. More info can be found in README.ti-secure

Right now, two image types are generated, MLO and X-LOADER. The types
are important, as certain boot modes implemented by the device's ROM
boot loader require one or the other (they are not equivalent). The
output filenames are u-boot-spl_HS_MLO and u-boot-spl_HS_X-LOADER. The
u-boot-spl_HS_MLO image is also copied to a file named MLO, which is
the name that the device ROM bootloader requires for loading from the
FAT partition of an SD card (same as on non-secure devices).

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>