Server object change

For review.

For clients, all servers information are taken from the LWM2M Security and Server Objects. These objects are thus mandatory.
The lib initialization is plit in two functions:
- lwm2m_init() to create a context. Parameters are two callbacks in change of the network communication.
The first one opens a communication session with a server identified by its ID. It is the user application responsability to retrieve associated info from the Securty object it provided. The core has no need for these info.
- lwm2m_configure() for clients to specify client informations and provide objects.

lwm2m_register() starts by parsing the LWM2M Security and Server Objects to create the lwm2m_context_t::serverList and lwm2m_context_t::bootstrapServerList before registering.

Bootstrap Servers are now stored as lwm2m_server_t in the lwm2m_context_t::bootstrapServerList.