samples: logging: set integration_platforms

Set integration_platforms on these samples to a single platform, we
prefer native_posix, than a qemu platform mps2_an385 and finally a
hardware platform.

For the 'basic' sample we use native_posix, for the 'rtt' we use
frdm_k64f ad we need a hardware platform that supports 'rtt', and for
the 'usermode' we utilize 'mps2_an385' as we can run in QEMU.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>