bootm: make use of legacy image format configurable

make the use of legacy image format configurable through
the config define CONFIG_IMAGE_FORMAT_LEGACY.

When relying on signed FIT images with required signature check
the legacy image format should be disabled. Therefore introduce
this new define and enable legacy image format if CONFIG_FIT_SIGNATURE
is not set. If CONFIG_FIT_SIGNATURE is set disable per default
the legacy image format.

Signed-off-by: Heiko Schocher <hs@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Cc: Lars Steubesand <lars.steubesand@philips.com>
Cc: Mike Pearce <mike@kaew.be>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Tom Rini <trini@ti.com>
Cc: Michal Simek <monstr@monstr.eu>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/README b/README
index 256d6ec..a248ab5 100644
--- a/README
+++ b/README
@@ -3200,6 +3200,19 @@
  -150	common/cmd_nand.c	Incorrect FIT image format
   151	common/cmd_nand.c	FIT image format OK
 
+- legacy image format:
+		CONFIG_IMAGE_FORMAT_LEGACY
+		enables the legacy image format support in U-Boot.
+
+		Default:
+		enabled if CONFIG_FIT_SIGNATURE is not defined.
+
+		CONFIG_DISABLE_IMAGE_LEGACY
+		disable the legacy image format
+
+		This define is introduced, as the legacy image format is
+		enabled per default for backward compatibility.
+
 - FIT image support:
 		CONFIG_FIT
 		Enable support for the FIT uImage format.
@@ -3216,6 +3229,11 @@
 		using a hash signed and verified using RSA. See
 		doc/uImage.FIT/signature.txt for more details.
 
+		WARNING: When relying on signed FIT images with required
+		signature check the legacy image format is default
+		disabled. If a board need legacy image format support
+		enable this through CONFIG_IMAGE_FORMAT_LEGACY
+
 - Standalone program support:
 		CONFIG_STANDALONE_LOAD_ADDR