cmake: zephyr toolchain code cleanup

With the revert of commit 820d327b4618d587a9cae0d085f00c6ab9c81cf2 then
some additional code can be cleaned up.

This removes the final left-overs from Zephyr SDK 0.11.1 support and
older.

It further aligns message printing when including Zephyr SDK toolchain
to other toolchain message printing.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>