Document using the U-Boot environment for TFTP+NFS booting?

[lucaceresoli]

In the yocto training lab (both STM32MP1 and BBB), we describe how to set up NFS boot via extlinux, and then the TFTP step is optional and not documented. However in the solutions we have a much quicker setup for TFTP+NFS using the u-boot environment (which is what I use, and AFAIK Alex does, too).

This is sometimes confusing for trainees doing the labs.

Should we use the U-Boot environment also in the labs?

[lucaceresoli]

Alex has some valid reasons to keep it as is: "it is longer to explain and onsite it is easier to use extlinux".