logging: Force LOG_PRINTK option if RTT backend is used

Log RTT backed is using mutex for locking access to RTT data.
RTT console (which by default is used by printk) writes to
RTT data directly and it cannot use mutex because it can be
called from any context (including interrupt). If both
modules access RTT buffer 0, data can be corrupted.

This patch forces LOG_PRINTK option if RTT backend is used to
ensure single point of access to RTT buffer 0 data.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>