This is the USB MSD boot code which should work on the Raspberry Pi model A, Compute Module, Compute module 3 and Raspberry Pi Zero.
This version of rpiboot has been modified to work from directories which contain the booting firmware. There is a default directory msd/ which contains bootcode.bin and start.elf to turn the Raspberry Pi device into a USB Mass Storage Device (MSD).
Clone this on your Pi or an Ubuntu linux machine
$ git clone --depth=1 https://github.com/raspberrypi/usbboot
$ cd usbboot
$ sudo apt install libusb-1.0-0-dev
$ make
$ sudo ./rpiboot
If you would like to boot the Raspberry Pi with a standard build you just need to copy the FAT partition files into a subdirectory (it must have at the minimum bootcode.bin and start.elf). If you take a standard firmware release then this will at the very least boot the linux kernel which will then stop (and possibly crash!) when it looks for a filesystem. To provide a filesystem there are many options, you can build an initramfs into the kernel, add an initramfs to the boot directory or provide some other interface to the filesystem.
$ sudo ./rpiboot -d boot
This will serve the boot directory to the Raspberry Pi Device.