Add --with-plugin-toolkit to README, clean up unwanted files

Change-Id: Ib4d1ee94706711939e03704c655ba355a8f82439
Signed-off-by: Dave Barach <dave@barachs.net>

build-root/emacs-lisp/README

@@ -66,7 +66,7 @@ Or, generate each file individually:
 
    $ mkdir build
    $ cd build
-   $ ../configure
+   $ ../configure --with-plugin-toolkit
    $ make
    $ sudo make install
 

build-root/emacs-lisp/plugin-makefile-skel.el

@@ -70,5 +70,7 @@ install-data-hook:
 	cp $(prefix)/lib/" plugin-name "_plugin.so.*.*.* /usr/lib/vpp_plugins
 	cp $(prefix)/lib/" plugin-name "_test_plugin.so.*.*.* \\
 		/usr/lib/vpp_api_test_plugins
+	rm -f $(prefix)/lib/" plugin-name "_plugin.*
+	rm -f $(prefix)/lib/" plugin-name "_test_plugin.*
 endif
 ")