arm: am4x: add U-Boot FIT signing and SPL image post-processing

Modify the SPL build procedure for AM437x high-security (HS) device
variants to create a secure u-boot_HS.img FIT blob that contains U-Boot
and DTB artifacts signed (and optionally encrypted) 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: Madan Srinivas <madans@ti.com>
Signed-off-by: Andreas Dannenberg <dannenberg@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/am43xx_hs_evm_defconfig b/configs/am43xx_hs_evm_defconfig
index 4856a19..68dfb6c 100644
--- a/configs/am43xx_hs_evm_defconfig
+++ b/configs/am43xx_hs_evm_defconfig
@@ -13,6 +13,7 @@
 CONFIG_FIT=y
 CONFIG_SYS_EXTRA_OPTIONS="CONS_INDEX=1, NAND"
 CONFIG_SPL_LOAD_FIT=y
+CONFIG_SPL_FIT_IMAGE_POST_PROCESS=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMLS is not set