kconfig: add CONFIG_SUPPORT_TPL

CONFIG_TPL should not be enabled for boards that do not have TPL.
CONFIG_SUPPORT_TPL introduced by this commit should be "select"ed
by boards with TPL support and CONFIG_TPL should depend on it.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/Kconfig b/Kconfig
index 29e0b92..932fc8b 100644
--- a/Kconfig
+++ b/Kconfig
@@ -73,6 +73,9 @@
 config SUPPORT_SPL
 	bool
 
+config SUPPORT_TPL
+	bool
+
 config SPL
 	bool
 	depends on SUPPORT_SPL
@@ -83,7 +86,7 @@
 
 config TPL
 	bool
-	depends on SPL
+	depends on SPL && SUPPORT_TPL
 	prompt "Enable TPL" if !SPL_BUILD
 	default y if TPL_BUILD
 	default n