logging: Fix RTT backend being enabled with shell RTT

If USE_SEGGER_RTT option was enabled then LOG_BACKEND_RTT was
enabled by default. In case shell RTT was also used then both
write to RTT. Updated Kconfig to prevent such situation.

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