shell: Add support to log_panic in RTT backend
shell: Add support to log_panic in RTT backend

Shell RTT backend was using RTT call which was using
lock (mutex). In case of panic, that lead to failure
if panic occured in the interrupt context where mutex
is not permitted.

Implementation changed to use write without log in
blocking mode.

Additionally, added static assert to check if raw RTT
log backend is not enabled on channel 0 which is used
by shell RTT backend.

Signed-off-by: Krzysztof Chruscinski <9b6dc988558c51b0405b49b55c8e66862969844b@nordicsemi.no>

Vudentz/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,ldts/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,finikorg/zephyr,finikorg/zephyr,Vudentz/zephyr,finikorg/zephyr,ldts/zephyr,explora26/zephyr,Vudentz/zephyr,Vudentz/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,Vudentz/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,finikorg/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,nashif/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,ldts/zephyr,galak/zephyr,explora26/zephyr,galak/zephyr,nashif/zephyr