__HAVE_ARCH_GENERIC_BOARD controls availabilty of generic board

We are introducing a new unified board setup. Add a check to make sure that
board config files do not define CONFIG_SYS_GENERIC_BOARD unless their
architecture defines __HAVE_ARCH_GENERIC_BOARD

__HAVE_ARCH_GENERIC_BOARD will currently not be the default setting, but
we can switch this later when most architecture support generic board.

Signed-off-by: Simon Glass <sjg@chromium.org>