nginx: introduce support for dynamic modules

nginx: introduce support for dynamic modules

Start building sub package that provide dynamic modules.

Each module needs to be loaded using load_modules.
Refer to nginx documentation on how to use this.

This should result in lower memory usage as only used module are loaded.

Also fix the uci-default scripts to add the required ubus module for
luci module.

-fvisibility=hidden is needed to be dropped to correctly support loading
dynamic modules.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>