Change wifi example to build only DISCO_L475VG_IOT01A

Currently mbed-os-example-blinky and mbed-os-example-wifi are built for each and every target in Mbed OS. This will change that only DISCO_L475VG_IOT01A is built for mbed-os-example-wifi. DISCO_L475VG_IOT01A was chosen as its one of the wi-fi supported boards and having integrated wi-fi.

Author: Jussi Siponen

tools/test/examples/examples.json

@@ -116,7 +116,7 @@
       "sub-repo-example": false,
       "subs": [],
       "features" : [],
-      "targets" : [],
+      "targets" : ["DISCO_L475VG_IOT01A"],
       "toolchains" : [],
       "exporters": [],
       "compile" : true,