devtool: modify: default source tree path

As per the changes to "devtool add", make the source tree path optional
and use the default path if none is specified.

(From OE-Core rev: 83707d1334fb094fd1877bcfd07a83866601048a)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/devtool

@@ -152,6 +152,10 @@ def _create_workspace(workspacedir, config, basepath):
             f.write('\nIf you no longer need to use devtool you can remove the path to this\n')
             f.write('workspace layer from your conf/bblayers.conf file (and then delete the\n')
             f.write('layer, if you wish).\n')
+            f.write('\nNote that by default, if devtool fetches and unpacks source code, it\n')
+            f.write('will place it in a subdirectory of a "sources" subdirectory of the\n')
+            f.write('layer. If you prefer it to be elsewhere you can specify the source\n')
+            f.write('tree path on the command line.\n')
 
 def _enable_workspace_layer(workspacedir, config, basepath):
     """Ensure the workspace layer is in bblayers.conf"""