net: openthread: Use different kconfig switch for shim and src

Different switch was used for build to allow including all shim body
in zephyr without the need for building openthread from this repo. This
allows developer to include custom OpenThread sources as part of the
application.
This change is needed as Thread is a subject of certification and
sources provided by zephyr may not necesairly pass certification or be
precertified. User is allowed to use certified OpenThread version
this way.

Signed-off-by: Marek Porwisz <marek.porwisz@nordicsemi.no>