Use initramfs on x86 but not on arm for factory install image.

build_image

 #!/bin/bash
 
 # Copyright (c) 2009 The Chromium OS Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # Script to build a bootable keyfob-based chromeos system image from within
 # a chromiumos setup. This assumes that all needed packages have been built into
 # the given target's root with binary packages turned on. This script will
 # build the Chrome OS image using only pre-built binary packages.
@@ skipping 172 matching lines @@
 fi
 
 OVERLAY_CHROMEOS_DIR="${SRC_ROOT}/third_party/chromiumos-overlay/chromeos/"
 
 # Determine build version.
 . "${OVERLAY_CHROMEOS_DIR}/config/chromeos_version.sh"
 
 # Configure extra USE or packages for this type of build.
 EXTRA_PACKAGES=""
 if [ ${FLAGS_factory_install} -eq ${FLAGS_TRUE} ] ; then
-  # Factory install needs to have the kernel initrmafs enabled,
-  # and the factory installer added.
+  # Factory install needs to have the factory installer added.
   EXTRA_PACKAGES="${EXTRA_PACKAGES} chromeos-base/chromeos-factoryinstall"
-  export USE="${USE} fbconsole initramfs"
+  # On x86, booting factory install shim on SD card needs to have the kernel
+  # initrmafs enabled. On ARM, booting factory install image on network does
+  # not needs initramfs. Force to enable fbconsole to fix a display driver bug.
+  if [ ${ARCH} -eq "arm" ] ; then
+    export USE="${USE} fbconsole"
+  else
+    export USE="${USE} initramfs"
+  fi
 fi
 
 emerge_to_image() {
   sudo -E ${EMERGE_BOARD_CMD} --root-deps=rdeps --usepkgonly \
     "$@" ${EMERGE_JOBS}
 }
 
 # Freshen kernel with correct USE flags. This is a noop if we have
 # the right kernel prebuilt. Factory install uses USE="initramfs".
 # We don't allow building from source with the image as a target,
@@ skipping 595 matching lines @@
   echo "Developer image created as ${DEVELOPER_IMAGE_NAME}"
 fi
 
 print_time_elapsed
 
 echo "To copy to USB keyfob, do something like:"
 echo "  ./image_to_usb.sh --from=${OUTSIDE_OUTPUT_DIR} --to=/dev/sdX"
 echo "To convert to VMWare image, INSIDE the chroot, do something like:"
 echo "  ./image_to_vm.sh --from=${OUTSIDE_OUTPUT_DIR} --board=${BOARD}"
 echo "from the scripts directory where you entered the chroot."