Magic incantation for Pine64 UEFI/PXE boot

[Ed Maste]

This seems to be somewhat under-documented, so for anyone else trying
to PXE boot a Pine64: after some trial and error I found "dhcp"
followed by "bootefi ${fileaddr} ${fdtcontroladdr}" works.

I used the most recent FreeBSD SD-card image (to provide U-boot), and
used standard configuration on the PXE boot host.

=> dhcp
BOOTP broadcast 1
DHCP client bound to address 10.0.0.101 (4 ms)
Using ethernet at 01c30000 device
TFTP from server 10.0.0.1; our IP address is 10.0.0.101
Filename 'arm64/boot/loader.efi'.
Load address: 0x42000000
Loading: *^H####################################
         840.8 KiB/s
done
Bytes transferred = 517120 (7e400 hex)
=> bootefi ${fileaddr} ${fdtcontroladdr}
## Starting EFI application at 42000000 ...
...