Only sign kernel for x86 for now. Leave ARM unsigned.

The signing work is being tested and developed on x86, and ARM isn't ready
to use it. Signing the ARM kernel is disruptive. We'll enable it for ARM
later.

Review URL: http://codereview.chromium.org/2599001

build_image

@@ -458,6 +458,10 @@ menuentry "local image B" {
 
 EOF
 
+# FIXME: At the moment, we're working on signed images for x86 only. ARM will
+# support this before shipping, but at the moment they don't.
+if [[ "$ARCH" = "x86" ]]; then
+  
   # Legacy BIOS will use the kernel in the rootfs (via syslinux), as will
   # standard EFI BIOS (via grub, from the EFI System Partition). Chrome OS BIOS
   # will use a separate signed kernel partition, which we'll create now.
@@ -495,6 +499,12 @@ kernel_utility --generate \
       --vmlinuz "${ROOT_FS_DIR}/boot/vmlinuz" \
       --out "${OUTPUT_DIR}/vmlinuz.image"
   
+else
+  # FIXME: For now, ARM just uses the unsigned kernel by itself.
+  cp -f "${ROOT_FS_DIR}/boot/vmlinuz" "${OUTPUT_DIR}/vmlinuz.image"
+fi
+
+
 # Perform any customizations on the root file system that are needed.
 "${SCRIPTS_DIR}/customize_rootfs" \
   --root="$ROOT_FS_DIR" \