ARM: ux500: add board autoselection

At least one platform needs to be selected, so pick the u8500
development board if nothing else is selected. The first
menu to choose the actual platform is meaningless and can be
made invisible.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>