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>

drivers/Makefile

@@ -10,6 +10,7 @@ obj-$(CONFIG_$(SPL_)RAM)	+= ram/
 
 ifdef CONFIG_SPL_BUILD
 
+obj-$(CONFIG_SPL_CRYPTO_SUPPORT) += crypto/
 obj-$(CONFIG_SPL_I2C_SUPPORT) += i2c/
 obj-$(CONFIG_SPL_GPIO_SUPPORT) += gpio/
 obj-$(CONFIG_SPL_MMC_SUPPORT) += mmc/

drivers/crypto/rsa_mod_exp/mod_exp_sw.c

@@ -32,6 +32,7 @@ U_BOOT_DRIVER(mod_exp_sw) = {
 	.name	= "mod_exp_sw",
 	.id	= UCLASS_MOD_EXP,
 	.ops	= &mod_exp_ops_sw,
+	.flags	= DM_FLAG_PRE_RELOC,
 };
 
 U_BOOT_DEVICE(mod_exp_sw) = {