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/common/cmd_source.c b/common/cmd_source.c
index 54ffd16..f3e9e60 100644
--- a/common/cmd_source.c
+++ b/common/cmd_source.c
@@ -29,7 +29,9 @@
 source (ulong addr, const char *fit_uname)
 {
 	ulong		len;
+#if defined(CONFIG_IMAGE_FORMAT_LEGACY)
 	const image_header_t *hdr;
+#endif
 	ulong		*data;
 	int		verify;
 	void *buf;
@@ -44,6 +46,7 @@
 
 	buf = map_sysmem(addr, 0);
 	switch (genimg_get_format(buf)) {
+#if defined(CONFIG_IMAGE_FORMAT_LEGACY)
 	case IMAGE_FORMAT_LEGACY:
 		hdr = buf;
 
@@ -84,6 +87,7 @@
 		 */
 		while (*data++);
 		break;
+#endif
 #if defined(CONFIG_FIT)
 	case IMAGE_FORMAT_FIT:
 		if (fit_uname == NULL) {