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/bootm.c b/cmd/bootm.c
index c3a0634..41b341e 100644
--- a/cmd/bootm.c
+++ b/cmd/bootm.c
@@ -249,7 +249,7 @@
 	printf("\n## Checking Image at %08lx ...\n", addr);
 
 	switch (genimg_get_format(hdr)) {
-#if defined(CONFIG_IMAGE_FORMAT_LEGACY)
+#if defined(CONFIG_LEGACY_IMAGE_FORMAT)
 	case IMAGE_FORMAT_LEGACY:
 		puts("   Legacy image found\n");
 		if (!image_check_magic(hdr)) {
@@ -337,7 +337,7 @@
 				goto next_sector;
 
 			switch (genimg_get_format(hdr)) {
-#if defined(CONFIG_IMAGE_FORMAT_LEGACY)
+#if defined(CONFIG_LEGACY_IMAGE_FORMAT)
 			case IMAGE_FORMAT_LEGACY:
 				if (!image_check_hcrc(hdr))
 					goto next_sector;
@@ -485,7 +485,7 @@
 			}
 
 			switch (genimg_get_format(buffer)) {
-#if defined(CONFIG_IMAGE_FORMAT_LEGACY)
+#if defined(CONFIG_LEGACY_IMAGE_FORMAT)
 			case IMAGE_FORMAT_LEGACY:
 				header = (const image_header_t *)buffer;