net: openthread: Add NCP start-up and required platform changes

When NCP starts-up some of the initialisation functions of a regular
OpenThread device do not need to be called, because they get triggered
by wpantund via UART. Instead NCP initialisation needs to be called.

A small typo has been fixed as well.

Also initialisation for raw link packet interface has been added. Can be
used for picking up 802.15.4 frames and interpreting them in the
application.

Signed-off-by: Markus Becker <markus.becker@tridonic.com>