Review: Update example_lib.py to use rmtree rather than rm system call.

Update examples.json to limit TLS examples to a couple of boards and only
GCC_ARM and ARM compilers.

tools/test/examples/examples.json

@@ -21,8 +21,8 @@
           "https://developer.mbed.org/teams/mbed-os-examples/code/mbed-os-example-tls-authcrypt"
       ],
       "features" : [],
-      "targets" : [],
-      "toolchains" : [],
+      "targets" : ["K64F", "NUCLEO_F429ZI"],
+      "toolchains" : ["GCC_ARM", "ARM"],
       "auto-update" : true
     },
     {

tools/test/examples/examples_lib.py

@@ -9,6 +9,7 @@ from os.path import dirname, abspath, basename
 import os.path
 import sys
 import subprocess
+from shutil import rmtree
 
 ROOT = abspath(dirname(dirname(dirname(dirname(__file__)))))
 sys.path.insert(0, ROOT)
@@ -116,7 +117,7 @@ def source_repos(config):
             name = basename(repo)
             if os.path.exists(name):
                 print("'%s' example directory already exists. Deleting..." % name)
-                subprocess.call(['rm', '-rf', name])
+                rmtree(name)
         
             subprocess.call(["mbed-cli", "import", repo])