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/common/image-fdt.c b/common/image-fdt.c
index eb552ca..e70da3d 100644
--- a/common/image-fdt.c
+++ b/common/image-fdt.c
@@ -33,7 +33,7 @@
 	puts(" - must RESET the board to recover.\n");
 }
 
-#if defined(CONFIG_IMAGE_FORMAT_LEGACY)
+#if CONFIG_IS_ENABLED(LEGACY_IMAGE_FORMAT)
 static const image_header_t *image_get_fdt(ulong fdt_addr)
 {
 	const image_header_t *fdt_hdr = map_sysmem(fdt_addr, 0);
@@ -263,7 +263,7 @@
 int boot_get_fdt(int flag, int argc, char * const argv[], uint8_t arch,
 		bootm_headers_t *images, char **of_flat_tree, ulong *of_size)
 {
-#if defined(CONFIG_IMAGE_FORMAT_LEGACY)
+#if CONFIG_IS_ENABLED(LEGACY_IMAGE_FORMAT)
 	const image_header_t *fdt_hdr;
 	ulong		load, load_end;
 	ulong		image_start, image_data, image_end;
@@ -344,7 +344,7 @@
 		 */
 		buf = map_sysmem(fdt_addr, 0);
 		switch (genimg_get_format(buf)) {
-#if defined(CONFIG_IMAGE_FORMAT_LEGACY)
+#if CONFIG_IS_ENABLED(LEGACY_IMAGE_FORMAT)
 		case IMAGE_FORMAT_LEGACY:
 			/* verify fdt_addr points to a valid image header */
 			printf("## Flattened Device Tree from Legacy Image at %08lx\n",