802.15.4 - Multipurpose frame support

[zephyrbot]

Reported by Gajinder Vij:

Add the means to parse or create Multipurpose frames.

(Imported from Jira ZEP-893)

[zephyrbot]

by ethan gao:

low-layer implementation detail and it makes more sense in unit test while developing

[zephyrbot]

by Andrei Laperie:

This is part of 2015 version of 802.15.4 standard. Currently it is not a requirement for anybody; we will schedule an implementation later when this changes.