cups: add/fix web interface packaging

cups: add/fix web interface packaging

cups includes a web server. Users can surf to port 631 (default) of a
machine running cups to (potentially, based on configuration, default off)
view jobs, add printers, and perform other forms of administration.

The location of the various resources that are used by the built-in web server
(e.g. index.html) are installed under ${datadir}/doc/cups. By default these
artifacts would be included in the ${PN}-doc package. The comments in this
recipe, however, would suggest an attempt was made to have them added to
${PN}; albeit unsuccessfully.

These resources add roughly 1.8M to an image.

Since cups does include a configuration option to disable the web interface
(--enable-webif), add a PACKAGECONFIG (default off) to allow the user to
decide whether or not they would like the web interface configured and its
pieces added to the image. Enabling this PACKAGECONFIG both enables the
web interface to be configured and built into cups, and also adds (by way
of a recommendation) the web interface package to the image. Considering
that the previous intention was not working, defaulting this option to off
preserves the existing behaviour. Previously in order to have the web
interface data included in an image, a user would have needed to explicitly
add the ${PN}-doc package to their image.

Signed-off-by: Trevor Woerner <twoerner@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>