Add the enable_rootfs_verification flag to cros_download_latest_image.

BUG=chromium-os:10993
TEST=Downloaded a latest image via cros_download_latest_image with
rootfs verification enabled and disabled; confirmed that the results
were as expected.

Change-Id: I489a20452bd29ac13fa45fec66214a045086634d
Reviewed-on: http://gerrit.chromium.org/gerrit/10405
Reviewed-by: Zelidrag Hornung <zelidrag@chromium.org>
Commit-Ready: Rahul Chaturvedi <rkc@chromium.org>
Tested-by: Rahul Chaturvedi <rkc@chromium.org>

bin/cros_download_latest_image

@@ -33,6 +33,8 @@ get_default_board
 DEFINE_string board "$DEFAULT_BOARD" \
   "The name of the board to check for images."
 DEFINE_boolean incremental "$FLAGS_FALSE" "Download incremental build"
+DEFINE_boolean enable_rootfs_verification "${FLAGS_TRUE}" \
+  "Enable rootfs verification for the downloaded image."
 
 # Parse command line flags.
 FLAGS "$@" || exit 1
@@ -66,5 +68,30 @@ if [ ! -e $LATEST_IMAGE_DIR/chromiumos_base_image.bin ]; then
       || die "Could not unzip image.zip"
 fi
 
+IMAGE_NAME=chromiumos_qemu_image.bin
+IMAGE_FILE="${LATEST_IMAGE_DIR}"/"${IMAGE_NAME}"
+ESP_DIR="${LATEST_IMAGE_DIR}/esp"
+
+cleanup() {
+  "$SCRIPTS_DIR/mount_gpt_image.sh" -u -e "${ESP_DIR}"
+}
+
+# This might seem a bit strange, but rootfs verification is already on with the
+# image, the only reason we've named this flag 'enable_rootfs_verfication'
+# versus 'disable_xyz' is to stay constant with the
+# 'noenable_rootfs_verification' flag used in build image to disable it.
+if [ "${FLAGS_enable_rootfs_verification}" -eq "${FLAGS_FALSE}" ] ; then
+  trap cleanup INT TERM EXIT
+  # We only need to touch the ESP part
+  "$SCRIPTS_DIR/mount_gpt_image.sh" -i chromiumos_qemu_image.bin \
+    -f "${LATEST_IMAGE_DIR}" -e "${ESP_DIR}"
+
+  # Change the default to boot from the usb instead of vusb, usb is set
+  # to boot from /dev/sda
+  sudo sed -i -e 's/vusb/usb/g' "${ESP_DIR}/syslinux/default.cfg"
+
+  sync; sync
+fi
+
 echo $LATEST_IMAGE_DIR
 exit 0