doc: Switch from setenv to env set

doc: Switch from setenv to env set

The "env" command is the recommended environment management command,
its "set" subcommand is the equivalent replacement for legacy "setenv"
command. Update the documentation to use the contemporary "env set"
command instead of legacy "setenv" command.

Note that the "setenv" command is unlikely to be removed from U-Boot
in the near future due to it being integral part of the command line
ABI.

Implemented using:
$ sed -i 's@\<setenv\>@env set@g' $(git grep -li '\<setenv\>' doc/) README

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>