shared/shell: Allow executing submenu commands

This makes it possible to execute submenu commands directly from the
main menu by using the format <submenu>.<command>:

[bluetooth]# advertise.uuids 0x1820
[bluetooth]# advertise.name blah
[bluetooth]# advertise on
Advertising object registered
UUID: Internet Protocol Support(0x1820)
Tx Power: off
LocalName: blah
Apperance: off
1 file changed