make themes: rename 'style' targtes to 'themes' targets

style.legacy could be renamed themes.legacy : it actually builds the files for
the legacy theme, then themes.legacy can be a dependency of themes.  Same for
the other styles.*

Debatable: about style.bootstrap, same convention : theme.bootstrap (even it is
more a toolbox for the oscar theme).

So there is no need to add the missing make styles in the help target.

thanks @dalf:

- https://github.com/asciimoo/searx/pull/1900#discussion_r399160355



Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>