configs: Rename CONFIG_IMAGE_FORMAT_LEGACY to CONFIG_LEGACY_IMAGE_FORMAT

The name CONFIG_LEGACY_IMAGE_FORMAT reads slightly better along with
allowing us to avoid a rather nasty Kbuild/Kconfig issue down the line
with CONFIG_IS_ENABLED(IMAGE_FORMAT_LEGACY).  In a few places outside of
cmd/ switch to using CONFIG_IS_ENABLED() to test what is set.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/include/image.h b/include/image.h
index bb7089e..5f82194 100644
--- a/include/image.h
+++ b/include/image.h
@@ -561,7 +561,7 @@
 #ifndef USE_HOSTCC
 /* Image format types, returned by _get_format() routine */
 #define IMAGE_FORMAT_INVALID	0x00
-#if defined(CONFIG_IMAGE_FORMAT_LEGACY)
+#if defined(CONFIG_LEGACY_IMAGE_FORMAT)
 #define IMAGE_FORMAT_LEGACY	0x01	/* legacy image_header based format */
 #endif
 #define IMAGE_FORMAT_FIT	0x02	/* new, libfdt based format */