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>

doc/develop/bootstd/overview.rst

@@ -152,7 +152,7 @@ boot_targets
 This environment variable can be used to control the list of bootdevs searched
 and their ordering, for example::
 
-   setenv boot_targets "mmc0 mmc1 usb pxe"
+   env set boot_targets "mmc0 mmc1 usb pxe"
 
 Entries may be removed or re-ordered in this list to affect the boot order. If
 the variable is empty, the default ordering is used, based on the priority of
@@ -169,7 +169,7 @@ used by the old distro scripts.
 This environment variable can be used to control the list of bootmeths used and
 their ordering for example::
 
-   setenv bootmeths "extlinux efi"
+   env set bootmeths "extlinux efi"
 
 Entries may be removed or re-ordered in this list to affect the order the
 bootmeths are tried on each bootdev. If the variable is empty, the default