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_fpga.c b/common/cmd_fpga.c
index bda5c8f..8c5bf44 100644
--- a/common/cmd_fpga.c
+++ b/common/cmd_fpga.c
@@ -201,6 +201,7 @@
 #if defined(CONFIG_CMD_FPGA_LOADMK)
 	case FPGA_LOADMK:
 		switch (genimg_get_format(fpga_data)) {
+#if defined(CONFIG_IMAGE_FORMAT_LEGACY)
 		case IMAGE_FORMAT_LEGACY:
 			{
 				image_header_t *hdr =
@@ -229,6 +230,7 @@
 					       BIT_FULL);
 			}
 			break;
+#endif
 #if defined(CONFIG_FIT)
 		case IMAGE_FORMAT_FIT:
 			{