Early log panic does not print logs on shell

[nordic-krch]

Describe the bug
When log_panic is triggered early,before log thread starts, no logs can be printed because shell acting as logger backend is not yet initialized.

To Reproduce
Modify samples/subsys/shell/shell_module. Add to main():

LOG_INF("test");
k_panic();

Observe on uart that panic report is printed but test log message isn't.

Expected behavior
All logs before log_panic() should be printed.

Impact
Early failures are hard to debug.

Additional context
If issue is difficult to solve a workaround would be to disable shell and enable standard UART log to debug early stage issue (see #12851).

[nordic-krch]

As i don't see possible way to fix it, decided that i'll document how to handle early crash.