ARM: uniphier: rename environment variable fdt_file to fdtfile

For booting Linux in the generic distro mechanism, cmd/pxe.c
retrieves the FDT file name from "fdtfile" environment variable.

Rename "fdt_file" to "fdtfile" for easier migration to distro boot.

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