zephyr: enable MULTITHREADING by default except on nRF

CONFIG_MULTITHREADING is generally assumed to be y by the zephyr
kernel, even though that's not reflected in the Kconfig. For example,
semaphores depend on it being y, but CMake will still build sem.c even
if it's n.

The sensible default for this option is thus y.

However, it's a size advantage to disable it if the platform drivers
are all aware of this option, such as nRF.

Let's re-work our use of this option to keep it disabled when we know
it's safe, but enable it by default so more platforms can be expected
to work out of the box.

Signed-off-by: Marti Bolivar <marti.bolivar@nordicsemi.no>