Please make NVMe boot work out of the box

[andreabolognani]

Despite the board having to built-in NVMe slot, it is possible to retrofit this functionality by purchasing an inexpensive adapter that fits into the PCIe slot.

NVMe support is compiled into U-Boot, so things mostly work already; however, a small tweak to the U-Boot environment is needed, specifically adding nvme0 to boot_targets as such:

=> env default -f -a
## Resetting to default environment

=> env print boot_targets
boot_targets=mmc1 usb sata mmc0

=> env set boot_targets "mmc1 usb nvme0 sata mmc0"; env print boot_targets                    
boot_targets=mmc1 usb nvme0 sata mmc0

=> env save
Saving Environment to SPIFlash... Erasing SPI flash...Writing to SPI flash...done
OK

I have documented these steps on the Fedora RISC-V wiki page dedicated to HiFive Premier P550, but it would be fantastic if the default environment came with this change already built in, making manual user intervention unnecessary.