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/disk.c b/cmd/disk.c
index dcc36a6..9e635c1 100644
--- a/cmd/disk.c
+++ b/cmd/disk.c
@@ -15,7 +15,7 @@
 	ulong addr = CONFIG_SYS_LOAD_ADDR;
 	ulong cnt;
 	disk_partition_t info;
-#if defined(CONFIG_IMAGE_FORMAT_LEGACY)
+#if defined(CONFIG_LEGACY_IMAGE_FORMAT)
 	image_header_t *hdr;
 #endif
 	struct blk_desc *dev_desc;
@@ -62,7 +62,7 @@
 	bootstage_mark(BOOTSTAGE_ID_IDE_PART_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;