Log Message:
- Update to u-boot-2019.04-rc4, which works much better on the
  MediaTek mt7623 SoC than the previous 2019.01-rc2.
- Pull down the necessary SDMMC binary headers and preloader from BPI's
  Github repository, and use them to assemble an image that can be written
  directly to an SD card before adding an MBR partition table.