spl: nand: support redundant u-boot image

On modern NAND it's more than recommended to have a backup copy of the
u-boot binary to recover from corruption: bitflips are quite common on
MLC NANDs, and the read-disturbance will corrupt your u-boot partitition
more quickly than what you would see on an SLC NAND.

Add an extra Kconfig option to specify the offset of the redundant u-boot
image.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>
[scottwood: added ifdef to fix build break]
Signed-off-by: Scott Wood <oss@buserror.net>

common/spl/spl_nand.c

@@ -134,6 +134,13 @@ int spl_nand_load_image(void)
 #endif
 	/* Load u-boot */
 	err = spl_nand_load_element(CONFIG_SYS_NAND_U_BOOT_OFFS, header);
+#ifdef CONFIG_SYS_NAND_U_BOOT_OFFS_REDUND
+#if CONFIG_SYS_NAND_U_BOOT_OFFS != CONFIG_SYS_NAND_U_BOOT_OFFS_REDUND
+	if (err)
+		err = spl_nand_load_element(CONFIG_SYS_NAND_U_BOOT_OFFS_REDUND,
+					    header);
+#endif
+#endif
 	nand_deselect();
 	return err;
 }

drivers/mtd/nand/Kconfig

@@ -117,6 +117,14 @@ config SYS_NAND_U_BOOT_OFFS
 	Set the offset from the start of the nand where u-boot should be
 	loaded from.
 
+config SYS_NAND_U_BOOT_OFFS_REDUND
+	hex "Location in NAND to read U-Boot from"
+	default SYS_NAND_U_BOOT_OFFS
+	depends on SYS_NAND_U_BOOT_LOCATIONS
+	help
+	Set the offset from the start of the nand where the redundant u-boot
+	should be loaded from.
+
 config SPL_NAND_DENALI
 	bool "Support Denali NAND controller for SPL"
 	help