kconfig: add CONFIG_SUPPORT_SPL

CONFIG_SPL should not be enabled for boards that do not have SPL.
CONFIG_SUPPORT_SPL introduced by this commit should be "select"ed
by boards with SPL support and CONFIG_SPL should depend on it.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/Kconfig b/Kconfig
index e0c8992..29e0b92 100644
--- a/Kconfig
+++ b/Kconfig
@@ -70,8 +70,12 @@
 	depends on $KCONFIG_OBJDIR="tpl"
 	default y
 
+config SUPPORT_SPL
+	bool
+
 config SPL
 	bool
+	depends on SUPPORT_SPL
 	prompt "Enable SPL" if !SPL_BUILD
 	default y if SPL_BUILD
 	help