Migrate IMAGE_FORMAT_LEGACY to Kconfig

This converts IMAGE_FORMAT_LEGACY to Kconfig

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/zynq_zc706_defconfig b/configs/zynq_zc706_defconfig
index dd1b1af..00042c1 100644
--- a/configs/zynq_zc706_defconfig
+++ b/configs/zynq_zc706_defconfig
@@ -11,6 +11,7 @@
 CONFIG_FIT=y
 CONFIG_FIT_SIGNATURE=y
 CONFIG_FIT_VERBOSE=y
+CONFIG_IMAGE_FORMAT_LEGACY=y
 CONFIG_BOOTCOMMAND="run $modeboot || run distro_bootcmd"
 # CONFIG_DISPLAY_CPUINFO is not set
 CONFIG_SPL_STACK_R=y