handler: add mechanism to generate external device handler stubs

handler: add mechanism to generate external device handler stubs

Parse JSON files in a given directory and pass the information on to a callback
function for creation of an external device handler stub.
The description contains:
 - 'name': the name of the device type,
 - 'ubus_name': the name of the external device handler daemon on ubus,
 - 'bridge': a flag indicating whether the devices are bridge-like,
 - optionally 'br_prefix': a prefix for created devices
   (only for bridge-like, defaults to type name),
 - 'config': the UCI config options for devices of this type, and
 - optionally 'info' and 'stats': the format of calls to info() and dump().

Signed-off-by: Arne Kappen <arne.kappen@hhi.fraunhofer.de>