kbuild: Allow for CONFIG_SYS_CONFIG_NAME to be unset

It is possible to have a platform which does not require a board.h file
to build, but today we need an empty one for our generated config.h file
to be valid. Allow for omitting this file if CONFIG_SYS_CONFIG_NAME is
not set.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/scripts/Makefile.autoconf b/scripts/Makefile.autoconf
index 0ade916..8208ffe 100644
--- a/scripts/Makefile.autoconf
+++ b/scripts/Makefile.autoconf
@@ -113,7 +113,7 @@
 define filechk_config_h
 	(echo "/* Automatically generated - do not edit */";		\
 	echo \#define CFG_BOARDDIR board/$(if $(VENDOR),$(VENDOR)/)$(BOARD);\
-	echo \#include \<configs/$(CONFIG_SYS_CONFIG_NAME).h\>;		\
+	$(if $(CONFIG_SYS_CONFIG_NAME),echo \#include \<configs/$(CONFIG_SYS_CONFIG_NAME).h\> ;) \
 	echo \#include \<asm/config.h\>;				\
 	echo \#include \<linux/kconfig.h\>;				\
 	echo \#include \<config_fallbacks.h\>;)