Migrate IMAGE_FORMAT_LEGACY to Kconfig

This converts IMAGE_FORMAT_LEGACY to Kconfig

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/Kconfig b/Kconfig
index 6957097..78afe72 100644
--- a/Kconfig
+++ b/Kconfig
@@ -361,6 +361,16 @@
 
 endif # FIT
 
+config IMAGE_FORMAT_LEGACY
+	bool "Enable support for the legacy image format"
+	default y if !FIT_SIGNATURE
+	help
+	  This option enables the legacy image format. It is enabled by
+	  default for backward compatibility, unless FIT_SIGNATURE is
+	  set where it is disabled so that unsigned images cannot be
+	  loaded. If a board needs the legacy image format support in this
+	  case, enable it here.
+
 config OF_BOARD_SETUP
 	bool "Set up board-specific details in device tree before boot"
 	depends on OF_LIBFDT