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/configs/linkit-smart-7688_defconfig b/configs/linkit-smart-7688_defconfig
index b3acbbc..3750e59 100644
--- a/configs/linkit-smart-7688_defconfig
+++ b/configs/linkit-smart-7688_defconfig
@@ -10,7 +10,7 @@
 CONFIG_MIPS_BOOT_FDT=y
 CONFIG_FIT=y
 CONFIG_FIT_SIGNATURE=y
-CONFIG_IMAGE_FORMAT_LEGACY=y
+CONFIG_LEGACY_IMAGE_FORMAT=y
 CONFIG_OF_STDOUT_VIA_ALIAS=y
 CONFIG_SYS_CONSOLE_INFO_QUIET=y
 CONFIG_BOARD_EARLY_INIT_F=y