ARM: uniphier: do not modify bootcmd environment variable at run-time

ARM: uniphier: do not modify bootcmd environment variable at run-time

Some users might want to modify 'bootcmd' at compile-time by editing
include/configs/uniphier.h, but overwriting it at run-time makes it
impossible.

Instead, set 'bootdev' at run-time, which contains the boot device the
system is booting from, then indirectly reference it from 'bootcmd'.

It is up to users whether to override 'bootcmd'.

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