stm32mp: stm32prog: solve compilation with CONFIG_FIT_SIGNATURE

When CONFIG_FIT_SIGNATURE is activated, CONFIG_LEGACY_IMAGE_FORMAT
is deactivated and the define IMAGE_FORMAT_LEGACY don't exist with
include/image.h:
  #if defined(CONFIG_LEGACY_IMAGE_FORMAT)
  #define IMAGE_FORMAT_LEGACY	0x01	/* legacy image_header based format */
  #endif

This patch adds the needed check on compilation flag
CONFIG_LEGACY_IMAGE_FORMAT to avoid the compilation error
for command stm32prog:

cmd_stm32prog.c:81:8: error: ‘IMAGE_FORMAT_LEGACY’ undeclared
(first use in this function); did you mean ‘IMAGE_FORMAT_FIT’?
   81 |    if (IMAGE_FORMAT_LEGACY ==
      |        ^~~~~~~~~~~~~~~~~~~
      |        IMAGE_FORMAT_FIT

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
diff --git a/arch/arm/mach-stm32mp/cmd_stm32prog/cmd_stm32prog.c b/arch/arm/mach-stm32mp/cmd_stm32prog/cmd_stm32prog.c
index e584bb5..f3790f9 100644
--- a/arch/arm/mach-stm32mp/cmd_stm32prog/cmd_stm32prog.c
+++ b/arch/arm/mach-stm32mp/cmd_stm32prog/cmd_stm32prog.c
@@ -77,10 +77,12 @@
 		if (header.type == HEADER_STM32IMAGE) {
 			size = header.image_length + BL_HEADER_SIZE;
 
+#if defined(CONFIG_LEGACY_IMAGE_FORMAT)
 			/* uImage detected in STM32IMAGE, execute the script */
 			if (IMAGE_FORMAT_LEGACY ==
 			    genimg_get_format((void *)(addr + BL_HEADER_SIZE)))
 				return image_source_script(addr + BL_HEADER_SIZE, "script@1");
+#endif
 		}
 	}