ARM: uniphier: make update commands more flexible for ATF

Currently, SPL (u-boot-spl.bin) and U-Boot (u-boot.bin) are stored
in non-volatile devices, and some environments are defined to update
the images easily.

When ARM Trusted Firmware is fully used, SPL is not used.  U-Boot
proper is contained as BL33 into FIP (Firmware Image Package), which
is standard container used by ATF.  Allow to use it.

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