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/source.c b/cmd/source.c
index 6d98a1c..1a9a71a 100644
--- a/cmd/source.c
+++ b/cmd/source.c
@@ -44,7 +44,7 @@
 source (ulong addr, const char *fit_uname)
 {
 	ulong		len;
-#if defined(CONFIG_IMAGE_FORMAT_LEGACY)
+#if defined(CONFIG_LEGACY_IMAGE_FORMAT)
 	const image_header_t *hdr;
 #endif
 	u32		*data;
@@ -61,7 +61,7 @@
 
 	buf = map_sysmem(addr, 0);
 	switch (genimg_get_format(buf)) {
-#if defined(CONFIG_IMAGE_FORMAT_LEGACY)
+#if defined(CONFIG_LEGACY_IMAGE_FORMAT)
 	case IMAGE_FORMAT_LEGACY:
 		hdr = buf;