ARM: UniPhier: add environment variable to update images in NAND

To boot UniPhier boards with the NAND boot mode, two images
(u-boot-spl.bin and u-boot-dtb.img) must be written at the correct
offset addresses.

TFTP downloading is useful to update such images in the NAND device.
We generally do:

  => nand erase 0 0x100000
  => tftpboot u-boot-spl.bin
  => nand write $loadaddr 0 0x10000
  => tftpboot u-boot-dtb.img
  => nand write $loadaddr 0x10000 0xf0000

It is a tedious and error-prone operation.

This commit provides the shorthand:

  => run nandupdate

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>

include/configs/uniphier.h

@@ -235,6 +235,11 @@
 	"image_offset=0x00080000\0"		\
 	"image_size=0x00f00000\0"		\
 	"verify=n\0"				\
+	"nandupdate=nand erase 0 0x100000 &&"				\
+		   "tftpboot u-boot-spl.bin &&"				\
+		   "nand write $loadaddr 0 0x10000 &&"			\
+		   "tftpboot u-boot-dtb.img &&"				\
+		   "nand write $loadaddr 0x10000 0xf0000\0"		\
 	"norboot=run add_default_bootargs &&"				\
 		"bootm $image_offset\0"					\
 	"nandboot=run add_default_bootargs &&"				\