Separate ONIX-specific string functions from main libONI library

[aacuevas]

Right now, the onix_device_str() and onix_hub_str coded in onix.c/h are embedded into the main libONI library. This hinders future developments as any new device created requires a build of libONI, which should be general-purpose.

We need to think of a better way to integrate those strings into client applications which is easy for the client developers but extensible.