Support for sub-resource opaque.

[joakimeriksson]

We need to support sub-resource opaques in the LWM2M engine. Suggested model is:

• Added argument for the enter / exit sub-resources with a struct that defines the sub-resource type, and an array of lengths (and the length of the array) so that it is possible to predict the total overhead for the data and generate a correct TLV.
• Export of opaque sub-resource TLV will be made by just exporting multiple opaques via the same opaque callback as used today. The Opaque callback will have to know when to export the next TLV.

[joakimeriksson]

We should likely also make a support-function for this - that makes life easy for anyone who needs to do a multi-instance resource opaque (or string).