rpi: use constant "unknown board" DT filename

rpi: use constant "unknown board" DT filename

To simplify support for new SoCs, just use a constant filename
for the unknown case. In practice this case shouldn't be hit anyway, so
the filename isn't relevant, and certainly doesn't need to differentiate
between SoCs. If a user has an as-yet-unknown board, they can override
this value in the environment anyway.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
Reviewed-by: Tom Rini <trini@konsulko.com>