Shell will store only pointer to the prompt string instead of
copying it to the RAM buffer. It will save RAM memory and
it will simplify implementation of a new feature: "select"
command. When a command will be selected than shell will
display command syntax as a prompt.
Removed obsolete ASSERT check in a static function.
Signed-off-by: Jakub Rzeszutko <jakub.rzeszutko@nordicsemi.no>