generic-board: move __HAVE_ARCH_GENERIC_BOARD to Kconfig

Move the option to Kconfig renaming it to CONFIG_HAVE_GENERIC_BOARD.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Alexey Brodkin <abrodkin@synopsys.com>