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>

README

@@ -401,7 +401,7 @@ The following options need to be configured:
 - USB Device:
 		Define the below if you wish to use the USB console.
 		Once firmware is rebuilt from a serial console issue the
-		command "setenv stdin usbtty; setenv stdout usbtty" and
+		command "env set stdin usbtty; env set stdout usbtty" and
 		attach your USB cable. The Unix command "dmesg" should print
 		it has found a new device. The environment variable usbtty
 		can be set to gserial or cdc_acm to enable your device to
@@ -725,7 +725,7 @@ The following options need to be configured:
 		remaining RAM in a form that can be passed as boot
 		argument to Linux, for instance like that:
 
-			setenv bootargs ... mem=\${mem}
+			env set bootargs ... mem=\${mem}
 			saveenv
 
 		This way you can tell Linux not to use this memory,
@@ -754,7 +754,7 @@ The following options need to be configured:
 		directly type `$name' at the command prompt.
 
 		Global environment variables are those you use
-		setenv/printenv to work with. To run a command stored
+		env set/printenv to work with. To run a command stored
 		in such a variable, you need to use the run command,
 		and you must not use the '$' sign to access them.
 
@@ -1655,13 +1655,13 @@ The "bootm" command is used to boot an application that is stored in
 memory (RAM or Flash). In case of a Linux kernel image, the contents
 of the "bootargs" environment variable is passed to the kernel as
 parameters. You can check and modify this variable using the
-"printenv" and "setenv" commands:
+"printenv" and "env set" commands:
 
 
 	=> printenv bootargs
 	bootargs=root=/dev/ram
 
-	=> setenv bootargs root=/dev/nfs rw nfsroot=10.0.0.2:/LinuxPPC nfsaddrs=10.0.0.99:10.0.0.2
+	=> env set bootargs root=/dev/nfs rw nfsroot=10.0.0.2:/LinuxPPC nfsaddrs=10.0.0.99:10.0.0.2
 
 	=> printenv bootargs
 	bootargs=root=/dev/nfs rw nfsroot=10.0.0.2:/LinuxPPC nfsaddrs=10.0.0.99:10.0.0.2