ARM: uniphier: enable distro boot

Switch to the distro boot for UniPhier platform.

 - Remove the environment vairalbes used to load images from raw
   block devices.

 - Keep the command to download images via tftp.  This will be
   useful to boot the kernel when no valid kernel image is ready
   yet in the file system.

 - Use root.cpio.gz instead of root.cpio.uboot because we always know
   the file size of the init ramdisk; it is loaded via either a file
   system or network.

 - Rename fit_addr_r to kernel_addr_r, which the distro command
   checks to get the load address of FIT image.

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