net: lwm2m: Allow initializing opaque and string data to zero length

By default, any string or opaque data that LwM2M engine initializes
sets data lenght to same value as given buffer length for that
resource.

However, on run time, engine keeps track how much data is written
to each resource, so when reading from any resource, should only
return data that has been written there. But uninitialized resources
return the content of the whole buffer.

Fixed the problem by introducing macros INIT_OBJ_RES_LEN(),
INIT_OBJ_RES_MULTI_DATA_LEN() and INIT_OBJ_RES_DATA_LEN() that
allows you to give the amount of data existing in buffer when
the resource is initialized. This sets the data_len and max_data_len
variables correctly.

Also introduced new functions lwm2m_engine_get_res_buf() and
lwm2m_engine_set_res_buf() that distinct between data size and
buffer size. Deprecated the previous functions
lwm2m_engine_get_res_data() and lwm2m_engine_set_res_data()

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>