NcpTypes: Enable conversion of new "node type" strings.
NcpTypes: Enable conversion of new "node type" strings.

This commits enhances the `string_to_node_type()` to allow some
more commonly used strings to be used as node type (e.g., "sed"
or "sleepy" for sleepy-end-device).

d-bahr/wpantund,openthread/wpantund,d-bahr/wpantund,abtink/wpantund,aeliot/wpantund,openthread/wpantund,abtink/wpantund,openthread/wpantund,aeliot/wpantund,abtink/wpantund,aeliot/wpantund,d-bahr/wpantund,aeliot/wpantund,openthread/wpantund,abtink/wpantund,d-bahr/wpantund