shell: kconfig: Have SHELL_BACKEND_RTT depend on USE_SEGGER_RTT

Have SHELL_BACKEND_RTT depend on USE_SEGGER_RTT instead of selecting it,
to fix some selects of USE_SEGGER_RTT with unsatisfied dependencies.
USE_SEGGER_RTT was being forced on without checking HAS_SEGGER_RTT.

This means configuration files now have to enable both USE_SEGGER_RTT
and SHELL_BACKEND_RTT to enable SHELL_BACKEND_RTT. At least
samples/subsys/shell/shell_module/prj_minimal_rtt.conf does.

(The original version of this commit added a dependency on
HAS_SEGGER_RTT to SHELL_BACKEND_RTT instead, but Krzysztof Chruscinski
suggested depending on USE_SEGGER_RTT instead.)

Footnote: 'depends on' conditions get propagated to properties, so this
change is the same as adding '... if USE_SEGGER_RTT' to all properties
(and the prompt).

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>