arm: omap5: add U-Boot FIT signing and SPL image post-processing

arm: omap5: add U-Boot FIT signing and SPL image post-processing

Modify the SPL build procedure for AM57xx and DRA7xx high-security (HS)
device variants to create a secure u-boot_HS.img FIT blob that contains
U-Boot and DTB artifacts signed with a TI-specific process based on the
CONFIG_TI_SECURE_DEVICE config option and the externally-provided image
signing tool.

Also populate the corresponding FIT image post processing call to be
performed during SPL runtime.

Signed-off-by: Daniel Allred <d-allred@ti.com>
Signed-off-by: Andreas Dannenberg <dannenberg@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>