pdu.h -- minimize change wrt. github/master branch

Only defines for

#define COAP_MEDIATYPE_APPLICATION_LWM2M_TEXT        1541 /* application/vnd.oma.lwm2m+text  */
#define COAP_MEDIATYPE_APPLICATION_LWM2M_TLV         1542 /* application/vnd.oma.lwm2m+tlv  */
#define COAP_MEDIATYPE_APPLICATION_LWM2M_JSON        1543 /* application/vnd.oma.lwm2m+json  */
#define COAP_MEDIATYPE_APPLICATION_LWM2M_OPAQUE      1544 /* application/vnd.oma.lwm2m+opaque  */

added.
Discuss if it is better to have them in a separate .h file (like pdu_phoenix.h) which is then included into pdu.h. That would minimize the change to one line (i.e., the resulting #include) and we would have all of our changes in one place.

@ban : what do you think.

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