colibri_t20: add update_uboot wrapper

Add universal update_uboot wrapper that helps to update
U-Boot image on internal storage(NAND).

Create an proper tegra image:
$ cbootimage -s tegra20 colibri_t20-512-v12-nand.img.cfg colibri_t20.img

Flash in U-boot shell:
> tftpboot ${loadaddr} ${board_name}/${board_name}.img
> run update_uboot

Signed-off-by: Igor Opaniuk <igor.opaniuk@toradex.com>
Reviewed-by: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>
diff --git a/include/configs/colibri_t20.h b/include/configs/colibri_t20.h
index 122294d..94e17bb 100644
--- a/include/configs/colibri_t20.h
+++ b/include/configs/colibri_t20.h
@@ -27,10 +27,14 @@
 #define CONFIG_TEGRA_NAND
 #define CONFIG_SYS_MAX_NAND_DEVICE	1
 
-/* Environment in NAND, 64K is a bit excessive but erase block is 512K anyway */
+#define UBOOT_UPDATE \
+	"update_uboot=nand erase.part u-boot && " \
+		"nand write ${loadaddr} u-boot ${filesize}\0" \
 
+/* Environment in NAND, 64K is a bit excessive but erase block is 512K anyway */
 #define BOARD_EXTRA_ENV_SETTINGS \
-	"mtdparts=" CONFIG_MTDPARTS_DEFAULT "\0"
+	"mtdparts=" CONFIG_MTDPARTS_DEFAULT "\0" \
+	UBOOT_UPDATE
 
 /* Increase console I/O buffer size */
 #undef CONFIG_SYS_CBSIZE