[cli] add platform command which returns current platform (#8444)

Sometime we need to identify which platform we are currently using,
especially in a test cases, where a unified interface is provided to
control devices, the test cases have no idea about current platform.

src/cli/README.md

@@ -85,6 +85,7 @@ Done
 - [parentpriority](#parentpriority)
 - [partitionid](#partitionid)
 - [ping](#ping-async--i-source-ipaddr-size-count-interval-hoplimit-timeout)
+- [platform](#platform)
 - [pollperiod](#pollperiod-pollperiod)
 - [preferrouterid](#preferrouterid-routerid)
 - [prefix](#prefix)
@@ -2336,6 +2337,16 @@ Stop sending ICMPv6 Echo Requests.
 Done
 ```
 
+### platform
+
+Print the current platform
+
+```bash
+> platform
+NRF52840
+Done
+```
+
 ### pollperiod
 
 Get the customized data poll period of sleepy end device (milliseconds). Only for certification test

src/cli/cli.cpp

@@ -5477,6 +5477,13 @@ exit:
 
 #endif // OPENTHREAD_CONFIG_PING_SENDER_ENABLE
 
+template <> otError Interpreter::Process<Cmd("platform")>(Arg aArgs[])
+{
+    OT_UNUSED_VARIABLE(aArgs);
+    OutputLine("%s", OPENTHREAD_CONFIG_PLATFORM_INFO);
+    return OT_ERROR_NONE;
+}
+
 template <> otError Interpreter::Process<Cmd("pollperiod")>(Arg aArgs[])
 {
     return ProcessGetSet(aArgs, otLinkGetPollPeriod, otLinkSetPollPeriod);
@@ -7335,6 +7342,7 @@ otError Interpreter::ProcessCommand(Arg aArgs[])
 #if OPENTHREAD_CONFIG_PING_SENDER_ENABLE
         CmdEntry("ping"),
 #endif
+        CmdEntry("platform"),
         CmdEntry("pollperiod"),
 #if OPENTHREAD_FTD
         CmdEntry("preferrouterid"),