Make it possible to use system mbedtls version

--- a/resource/csdk/connectivity/src/SConscript
+++ b/resource/csdk/connectivity/src/SConscript
@@ -73,16 +73,7 @@ if env.get('SECURED') == '1':
 	env.AppendUnique(CPPPATH = [src_dir + '/resource/csdk/security/include'])
 	env.AppendUnique(CPPDEFINES = ['__WITH_DTLS__'])
 	env.AppendUnique(CPPPATH = [os.path.join(root_dir, 'external/inc')])
-	tls_path = 'extlibs/mbedtls';
-	tls_headers_path = 'mbedtls/include';
-	if ca_os == 'tizen' and os.path.exists(root_dir + '/' + tls_path):
-		env.SConscript(os.path.join(root_dir, tls_path + '/SConscript'))
-		env.AppendUnique(CPPPATH = [os.path.join(root_dir, tls_path + '/' + tls_headers_path)])
-	else:
-		if ca_os != 'android':
-			env.SConscript('#' + tls_path + '/SConscript')
-		env.AppendUnique(CPPPATH = ['#' + tls_path + '/' + tls_headers_path])
-		env.AppendUnique(CA_SRC = [os.path.join(ca_path, 'adapter_util/ca_adapter_net_ssl.c')])
+	env.AppendUnique(CA_SRC = [os.path.join(ca_path, 'adapter_util/ca_adapter_net_ssl.c')])
 
 if ((secured == '1') and (with_tcp == True)):
 	env.AppendUnique(CPPDEFINES = ['__WITH_TLS__'])