sys/console: Make echo of LF optional in silenced NLIP

Code was echoing back LF character even when application
requested to silence non-NLIP data on input.
Now this behaviour is still possible but require explicit
setting CONSOLE_NLIP_ECHO_LF which is disabled by default.

sys/console/full/src/console.c

@@ -1021,8 +1021,8 @@ handle_nlip(uint8_t byte)
         } else if (byte == CONSOLE_NLIP_PKT_START1) {
             nlip_state = NLIP_PKT_START1;
         } else {
-            /* For old code compatibility end of lines characters pass through */
-            handled = g_console_ignore_non_nlip && byte != '\r' && byte != '\n';
+            handled = g_console_ignore_non_nlip &&
+                !MYNEWT_VAL(CONSOLE_NLIP_ECHO_LF) && byte != '\r' && byte != '\n';
         }
         break;
     }

sys/console/full/syscfg.yml

@@ -38,6 +38,13 @@ syscfg.defs:
     CONSOLE_NLIP:
         description: 'Enable NLIP handling in console code'
         value: 1
+    CONSOLE_NLIP_ECHO_LF:
+        description: >
+            Enable LF echo even if function console_ignore_non_nlip()
+            was called to ignore any non NLIP input. This allows for
+            simple check if application is running correctly
+            by using simple terminal.
+        value: 0
     CONSOLE_COMPAT:
         description: 'Console backward compatibility'
         value: 1