arm: allow CONFIG_LINUX_KERNEL_IMAGE_HEADER to be set in defconfig

Let this option be enabled dynamically for boards that support multiple
mechanisms for booting U-Boot (e.g. as a first-stage or chainloaded
bootloader).

Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>
---
This patch has no dependencies

Cc: Simon Glass <sjg@chromium.org>
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index bd48131..1add428 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -81,7 +81,7 @@
 
 config LINUX_KERNEL_IMAGE_HEADER
 	depends on ARM64
-	bool
+	bool "Linux kernel binary header"
 	help
 	  Place a Linux kernel image header at the start of the U-Boot binary.
 	  The format of the header is described in the Linux kernel source at