This bug is fix in 2018b version.
With the latest version of matlab, we can share model configuration with referenced model and generate code.
The function interface is no outside the configuration.
So as it is independant of the model configuration it is possible to share it.
To find the function interface configuration you shall now search a bite as it is well hiden.
This can be access throught: Configure Model in Code Peerspective -> Code Mappings -C -> Entry-Point Functions.
Then in function name column use the 3 dots (verticals) to open the function interface panel that is quite the same as before.