colibri_vf: 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_vf.h

@@ -58,6 +58,10 @@
 	"ramdisk_addr_r=0x82100000\0" \
 	"scriptaddr=0x87000000\0"
 
+#define UBOOT_UPDATE \
+	"update_uboot=nand erase.part u-boot && " \
+		"nand write ${loadaddr} u-boot ${filesize}\0" \
+
 #define NFS_BOOTCMD \
 	"nfsargs=ip=:::::eth0: root=/dev/nfs\0"	\
 	"nfsboot=run setup; " \
@@ -112,6 +116,7 @@
 	NFS_BOOTCMD \
 	SD_BOOTCMD \
 	UBI_BOOTCMD \
+	UBOOT_UPDATE \
 	"console=ttyLP0\0" \
 	"defargs=user_debug=30\0" \
 	"dfu_alt_info=" DFU_ALT_NAND_INFO "\0" \