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/gardena-smart-gateway-mt7688-ram_defconfig b/configs/gardena-smart-gateway-mt7688-ram_defconfig
index 4edade4..1a1167b 100644
--- a/configs/gardena-smart-gateway-mt7688-ram_defconfig
+++ b/configs/gardena-smart-gateway-mt7688-ram_defconfig
@@ -9,7 +9,7 @@
 CONFIG_ENV_VARS_UBOOT_CONFIG=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_USE_BOOTCOMMAND=y
 CONFIG_BOOTCOMMAND="cp.b 83000000 84000000 10000 && dhcp uEnv.txt && env import -t ${fileaddr} ${filesize} && run do_u_boot_init; reset"