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>
diff --git a/drivers/mtd/nand/Kconfig b/drivers/mtd/nand/Kconfig
index 5fe169f..8c46a2f 100644
--- a/drivers/mtd/nand/Kconfig
+++ b/drivers/mtd/nand/Kconfig
@@ -117,6 +117,14 @@
 	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