Mainline U-Boot with SPI, NVMe and SATA boot support

[Bullet64]

I have updated the OP with info about a new release, based on mainline U-Boot v2020.01, and now with PCIe SATA drive boot support as well as NVMe.

There are also patches on the U-Boot mailing list which get HDMI output working from U-Boot. I've tested this and it works as expected, but I've not included it in the current build because

• Using HDMI output from U-Boot seems to break HDMI output from mainline Linux until a Linux app runs that changes the video mode, which resets HDMI and gets it working again.
  
• to be properly useful to interact with U-Boot, it'd require a USB keyboard, but starting U-Boot's USB support (required before any keyboard is recognised) takes several seconds, and it doesn't seem worthwhile to slow down every boot with the "usb start" command when normally it's not required.
  

In any case, if anyone is interested, you can see the changes required for HDMI and USB keyboard support in this branch.

Will test this later. Thank you!

Edit: sata boot works as expected