Beaglebone Black: Boot from USB or serial

We are developing a board based on the TI AM3358 processor and we are using the Beaglebone Black as a development test platform prior to the real hardware becoming available.
Our system will have eMMC memory but no microSD card so we intend to do a boot/install via the USB or serial port.
We are testing this out using the Beaglebone Black but it is not working.
We can boot our own Linux from the microSD card by powering up with the S2 boot button pressed fine.
However if we power up (via a USB-B cable) without a microSD card and the S2 boot button pressed the BBB does not seem to try and boot via the USB or serial.
Our host is Linux and a “lsusb” shows no new devices (We were expecting an Ethernet gadget device). Minicom or a raw serial port program connected to UART0 shows no ASIC id (what ever that is) being sent.
Any ideas ?

Here you have src code to do a usb boot/flash the eMMC with an image in .tar.xz format as I remember.

When the ROM USB boot kicks in you will see an ethernet gadget device, you can find the vid/pid in my code over there. Check if your linux machine has rndis drivers because I had this problem with my Android device, did not have rndis drivers and will not enumerate the device at all. Put a RS232 cable to the board to see if it really goes into USB boot you should receive some awkward chars on the terminal.