The BCM20732S module has an EEPROM built into it. The application code needs to be programmed into this in the factory (using the HCI/factory UART pins). The app should configure the SPIFFY2 interface as a slave and this is what you need to use to interface with the external microprocessor.
> Its not clear from the documentation whether the SPIFFY2 is bootable into this mode without the module having been pre-programmed with firmware written by the customer, so please can you elaborate?
The SoC always boots off the EEPROM built into the BCM20732S module.