logging: Handle panic occuring before log initialization

Log backends (marked as autostart) are initialized late. By default
in logger thread which has the lowest priority. If log_panic() occurs
earlier no logs is printed because there is no backend enabled.

This patch fixes it by adding log_init() call to log_panic(). Log_init()
can be called multiple times.

This patch ensures that logs are printed if early panic occurs if
backend is configured to auto-start. This is not the case if shell
is acting as log backend.

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