logging: Enable LOG_PRINTK if PRINTK is enabled

This change will enable the logging system to handle printk() calls by
default if printk support is enabled, which improves output resiliency
when logging, mcumgr and printk calls are used. Will disable keep
LOG_PRINTK disabled for tests which are not using the new API as
moving to LOG_PRINTK causes test data to be output to a different UART
in the old test system.

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>