shell: add getopt library support

This functionality is is enabled by setting CONFIG_SHELL_GETOPT.
It is not active by default.

User can call following functions inside command handlers:
 - shell_getopt - getopt function based on freebsd implementation
 - shell_getopt_status_get - returns getopt status

Beware when getopt functionality is enabled shell will not parse
command handler to look for "-h" or "--help" options and print
help message automatically.

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