ARM: uniphier: add sdscript, sdboot, sdupdate environment variables

Add handy macros:

  - sdscript: source boot.scr in the file system of the SD media
  - sdboot  : boot the kernel using the images in the file system
              of the SD media
  - sdscript: update the boot firmware in the SD media
              (in raw block sectors)

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>