shell: improve partial completion

Shell used to require adding commands and subcommands in alphabetical
order to ensure correct autocompletion and printing options with the
<Tab> key.

Commit: bd3a4e5f implemented correct
options printing with the <Tab> key for not sorted commands.
This PR allows to partially autocomplete not sorted commands.

Signed-off-by: Jakub Rzeszutko <jakub.rzeszutko@nordicsemi.no>