Installing 12c and get invalid inventory pointer location file

This happened to me recently when testing a 12.1.0.2 upgrade 
As I know for sure this machine's inventory is good I decided to investigate. 

Metalink Note 1292347.1 is a useful reference 

It turns out the inventory oraInst.loc file used to be able to get away with only inst_loc line , now you need inst_group as well. 

on Linux this is usually oinstall. 

So oraInst.loc should look like 

inventory_loc=/u01/oracle/inventory

inst_group=oinstall

Once that is done re-run the installer and it should now start up OK.