message: [nit] fix BACKBONE_ROUTER readonly warning
diff: @@ -31,7 +31,6 @@ readonly OTBR_TOP_BUILDDIR="${BUILD_DIR}/otbr"
  readonly OTBR_TOP_SRCDIR="$PWD"
  readonly OTBR_OPTIONS="${OTBR_OPTIONS:-}"
  readonly REFERENCE_DEVICE="${REFERENCE_DEVICE:-0}"
  -readonly BACKBONE_ROUTER="${BACKBONE_ROUTER:-0}"
  otbr_uninstall()
  {
  @@ -76,7 +75,7 @@ otbr_install()
  "-DOT_REFERENCE_DEVICE=ON"
  )
  fi
  - if [[ ${BACKBONE_ROUTER} == "1" ]]; then
  + if with BACKBONE_ROUTER; then
  otbr_options+=(
  "-DOTBR_BACKBONE_ROUTER=ON"
  )
  