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>
diff --git a/Makefile b/Makefile
index 346ea27..0d160c9 100644
--- a/Makefile
+++ b/Makefile
@@ -1163,7 +1163,7 @@
 
 prepare1: prepare2 $(version_h) $(timestamp_h) \
                    include/config/auto.conf
-ifeq ($(__HAVE_ARCH_GENERIC_BOARD),)
+ifeq ($(CONFIG_HAVE_GENERIC_BOARD),)
 ifeq ($(CONFIG_SYS_GENERIC_BOARD),y)
 	@echo >&2 "  Your architecture does not support generic board."
 	@echo >&2 "  Please undefine CONFIG_SYS_GENERIC_BOARD in your board config file."