Add two utility functions: lwm2m_tlv_encode_int() and lwm2m_tlv_decode_int().

Signed-off-by: David Navarro <david.navarro@intel.com>
API Change: Modify object callbacks

Hide the TLV complexity from the objects.
Checks for instance existence is done in the core.
Also fixes some small bugs.

Signed-off-by: David Navarro <david.navarro@intel.com>