Beaglebone black: Boot from USB or serial

We are designing a system based on the TI AM3358 processor and are using a Beaglebone black as a development board prior to getting the real hardware.
The real hardware will have eMMC memory but will not have a microSD card socket so for a bare metal boot/install we plan to use USB or serial.
We have been trying to do this with our Beaglebone black, but without success.
We can boot from our own Linux image on a microSD card by holding down the S2 button and powering up fine.
If we remove the microSD card and power up (via a mini USB-B cable plugged into the mini USB socket) with S2 pressed the system does not boot, as expected, but does not appear to try and boot via USB or serial.
We are using a Linux host system and the command “lsusb” shows no extra devices, we were expecting an Ethernet Gadget device. A minicom (or raw program) connection to the serial port (UART0) doesn’t show any output, we were expecting an ASIC id (whatever that looks like).
Any idea why this is not working ?