recipetool/create_buildsys_python: fix license note

recipetool/create_buildsys_python: fix license note

License field of setup is not always standardized, so we usually use the
classifier to determine the correct license format to use in the recipe.

A warning note is added above the LICENSE field of the create recipe
in case a license is provided in setup. But when the plugin is called,
"LICENSE =" is not yet present so we can never display this note.
Replace the "LICENSE =" condition with "##LICENSE_PLACEHOLDER##"
to actually be able to display the note message

Signed-off-by: Julien Stephan <jstephan@baylibre.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>