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/cmd/nand.c b/cmd/nand.c
index a22945d..899d504 100644
--- a/cmd/nand.c
+++ b/cmd/nand.c
@@ -846,7 +846,7 @@
 	int r;
 	char *s;
 	size_t cnt;
-#if defined(CONFIG_IMAGE_FORMAT_LEGACY)
+#if defined(CONFIG_LEGACY_IMAGE_FORMAT)
 	image_header_t *hdr;
 #endif
 #if defined(CONFIG_FIT)
@@ -874,7 +874,7 @@
 	bootstage_mark(BOOTSTAGE_ID_NAND_HDR_READ);
 
 	switch (genimg_get_format ((void *)addr)) {
-#if defined(CONFIG_IMAGE_FORMAT_LEGACY)
+#if defined(CONFIG_LEGACY_IMAGE_FORMAT)
 	case IMAGE_FORMAT_LEGACY:
 		hdr = (image_header_t *)addr;