Bluetooth: Mesh: Warn log mode incompatibility with friendship

Log modes other than the deferred may cause unintended delays during
processing of log messages. This in turns will affect scheduling of the
receive delay and receive window for both, Friend and LPN.

Adding a compilation warning instead of assert as a user may still want
to compile a code with friendship feature enabled without actually
establishing a friendship.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>