shell: Postpone enabling log backend

Shell log backend was enabled too early, before shell thread was
up and running. That could lead to failure if log thread passes
log message to shell log backend.

Additionally, modified algorithm for multiplexing log output with
shell prompt. In case of logs flood prompt was unreadable because
it was deleted immediately after being printed. Added k_sleep
after printing prompt if any character was typed. This ensures that
shell prompt is readable if user uses it.

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