File _multibuild of Package u-boot

<multibuild>
  <flavor>pinebook-pro-rk3399</flavor>
  <flavor>rock-pi-4-rk3399</flavor>
  <flavor>rock64-rk3328</flavor>
  <flavor>rockpro64-rk3399</flavor>
  <flavor>sige7-rk3588</flavor>
</multibuild>