colibri-imx6ull: add update_uboot wrapper

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

Usage example:
> tftpboot ${loadaddr} ${board_name}/u-boot-nand.imx
> run update_uboot
Signed-off-by: NIgor Opaniuk <igor.opaniuk@toradex.com>

include/configs/colibri-imx6ull.h

@@ -47,6 +47,12 @@
 	"ramdisk_addr_r=0x82200000\0" \
 	"scriptaddr=0x87000000\0"
 
+#define UBOOT_UPDATE \
+	"update_uboot=nand erase.part u-boot1 && " \
+		"nand write ${loadaddr} u-boot1 ${filesize} && " \
+		"nand erase.part u-boot2 && " \
+		"nand write ${loadaddr} u-boot2 ${filesize}\0"
+
 #define NFS_BOOTCMD \
 	"nfsargs=ip=:::::eth0: root=/dev/nfs\0" \
 	"nfsboot=run setup; " \
@@ -83,6 +89,7 @@
 	MEM_LAYOUT_ENV_SETTINGS \
 	NFS_BOOTCMD \
 	UBI_BOOTCMD \
+	UBOOT_UPDATE \
 	"console=ttymxc0\0" \
 	"defargs=user_debug=30\0" \
 	"dfu_alt_info=" DFU_ALT_NAND_INFO "\0" \