verified-boot: Minimal support for booting U-Boot proper from SPL

This allows a board to configure verified boot within the SPL using
a FIT or FIT with external data. It also allows the SPL to perform
signature verification without needing relocation.

The board configuration will need to add the following feature defines:
CONFIG_SPL_CRYPTO_SUPPORT
CONFIG_SPL_HASH_SUPPORT
CONFIG_SPL_SHA256

In this example, SHA256 is the only selected hashing algorithm.

And the following booleans:
CONFIG_SPL=y
CONFIG_SPL_DM=y
CONFIG_SPL_LOAD_FIT=y
CONFIG_SPL_FIT=y
CONFIG_SPL_OF_CONTROL=y
CONFIG_SPL_OF_LIBFDT=y
CONFIG_SPL_FIT_SIGNATURE=y

Signed-off-by: Teddy Reed <teddy.reed@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>
Acked-by: Andreas Dannenberg <dannenberg@ti.com>
Acked-by: Sumit Garg <sumit.garg@nxp.com>

Kconfig

@@ -183,6 +183,11 @@ config FIT
 	  verified boot (secure boot using RSA). This option enables that
 	  feature.
 
+config SPL_FIT
+	bool "Support Flattened Image Tree within SPL"
+	depends on FIT
+	depends on SPL
+
 config FIT_VERBOSE
 	bool "Display verbose messages on FIT boot"
 	depends on FIT
@@ -205,6 +210,12 @@ config FIT_SIGNATURE
 	  format support in this case, enable it using
 	  CONFIG_IMAGE_FORMAT_LEGACY.
 
+config SPL_FIT_SIGNATURE
+	bool "Enable signature verification of FIT firmware within SPL"
+	depends on SPL_FIT
+	depends on SPL_DM
+	select SPL_RSA
+
 config FIT_BEST_MATCH
 	bool "Select the best match for the kernel device tree"
 	depends on FIT