I installed unixodbc and unixodbc-dev through apt-get. I then installed oracle instantclient (both the basic and the odbc drivers). Now I am trying to run odbc_update_ini.sh but none of the paths I have tried for the ODBCDM_Home argument seem to be correct. On OSX where I installed unixodbc with brew I was able to use /usr/local/Cellar/unixodbc/2.3.4/ to make things work correctly. But I cannot figure out the ubuntu/apt-get equivalent.
I always change odbc.ini and odbcinst.ini manually. If you really want to use this odbc_update_ini.sh utility I would suggest to analyze its source code in order to check the path of the files it uses.
I won't be surprised if you need to set ODBCDM_Home to the root (/) directory.