samples: Use immediate logging mode in watchdog related samples

Starting with commit b9b43a0eb772a464bba13833d11e3a31fbf4e09e, printk()
messages are handled by the logging subsystem. This can cause trouble
in watchdog related samples if the deferred logging mode is used (and
currently it is by default), because those samples are ended by a reset
and some messages may not get a chance to be outputted.
Since the same problem concerns also the ordinary logging messages that
may be produced during execution of the samples, this commit fixes it
by switching to the immediate logging mode, not by just disabling the
LOG_PRINTK option.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>