nginx-dav-ext: new package

nginx-dav-ext: new package

Nginx built-in support for webdav is missing support for two commands:
PROPFIND and OPTIONS. This commit adds a new package that provides an
external nginx module with improved webdav support.

Signed-off-by: Johan Oudinet <johan.oudinet@gmail.com>
[Thomas:
 - Remove the BR2_PACKAGE_NGINX_HTTP_DAV_EXT_MODULE sub-option of the
   nginx package. The BR2_PACKAGE_NGINX_DAV_EXT option is sufficient.
 - Move the nginx.mk code together with another external module being
   enabled, nginx-upload.
 - Add LICENSE and LICENSE_FILES variables.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>