shell: kconfig: Put 'menuconfig SHELL' in top-level menu

The 'Shell Options' menu contains just the 'menuconfig SHELL' menu in
the menuconfig interface.

Remove the 'Shell Options' menu and put 'menuconfig SHELL' directly in
the top-level menu instead. Also change the prompt from "Enable shell"
to just "Shell", to make it consistent with "Logging".

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