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/Kconfig b/Kconfig
index df8f294..d2eb744 100644
--- a/Kconfig
+++ b/Kconfig
@@ -344,7 +344,7 @@
 	  check the legacy image format is disabled by default, so that
 	  unsigned images cannot be loaded. If a board needs the legacy image
 	  format support in this case, enable it using
-	  CONFIG_IMAGE_FORMAT_LEGACY.
+	  CONFIG_LEGACY_IMAGE_FORMAT.
 
 config FIT_SIGNATURE_MAX_SIZE
 	hex "Max size of signed FIT structures"
@@ -473,7 +473,7 @@
 
 endif # FIT
 
-config IMAGE_FORMAT_LEGACY
+config LEGACY_IMAGE_FORMAT
 	bool "Enable support for the legacy image format"
 	default y if !FIT_SIGNATURE
 	help