uboot-phytec

arc: arcangel4: set board entry <none> to fix a build error

There are no source files in board/synopsys/arcangel4/
directory.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Alexey Brodkin <Alexey.Brodkin@synopsys.com>