nghttp2: rewrite recipe to be an idiomatic library recipe

nghttp2: rewrite recipe to be an idiomatic library recipe

This recipe for nghttp2 doesn't build any of the binaries, just the core
library, but is structured like a recipe that is primarily an application
that happens to ship libraries.

Remove the lib${BPN} package and put the library into PN (which will then
be debian-renamed).

Use the shorthand option to just build the library.

Add documentation enabling/disabling options so we don't install the
docs if not needed. Currently there are no extra dependencies as the
sphinx-generated manpages are pre-built in the tarballs, but this could
change.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>