build: Get rid of scary /dev/sdb defaults in scripts.

src/scripts/build_image

Index: src/scripts/build_image
diff --git a/src/scripts/build_image b/src/scripts/build_image
index c93ed350b6122d5504986971bbc4e9022f504bd4..a7f2366382ca796e7523b1cfd09a187b86f7e500 100755
--- a/src/scripts/build_image
+++ b/src/scripts/build_image
@@ -432,7 +432,7 @@ rmdir "${ROOT_FS_DIR}" "${STATEFUL_DIR}" "${ESP_DIR}"
 OUTSIDE_OUTPUT_DIR="../build/images/${FLAGS_board}/${IMAGE_SUBDIR}"
 echo "Done.  Image created in ${OUTPUT_DIR}"
 echo "To copy to USB keyfob, OUTSIDE the chroot, do something like:"
-echo "  ./image_to_usb.sh --from=${OUTSIDE_OUTPUT_DIR} --to=/dev/sdb"
+echo "  ./image_to_usb.sh --from=${OUTSIDE_OUTPUT_DIR} --to=/dev/sdX"
 echo "To convert to VMWare image, OUTSIDE the chroot, do something like:"
 echo "  ./image_to_vmware.sh --from=${OUTSIDE_OUTPUT_DIR}"
 echo "from the scripts directory where you entered the chroot."