bluetooth: Allow immediate logging with software-based LL
bluetooth: Allow immediate logging with software-based LL

Some logging configuration are compatible with software-base Link
Layer. Currently only one configuration is supported:
RTT back-end in drop mode and no clean output

Signed-off-by: Guillaume Lager <73a137297c67d2c3a8f88db4b715c3321d1681be@gmail.com>

Vudentz/zephyr,nashif/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,finikorg/zephyr,Vudentz/zephyr,Vudentz/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,Vudentz/zephyr,nashif/zephyr,nashif/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,nashif/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr