This is the USB MSD boot code which should work on the Raspberry Pi model A, Compute Module, Compute Module 3, Compute Module 4 and Raspberry Pi Zero.
This version of rpiboot has been modified to work from directories which contain the booting firmware. There is a msd/ directory which contains bootcode.bin and start.elf to turn the Raspberry Pi device into a USB Mass Storage Device (MSD). If run without arguments embedded versions of bootcode.bin and start.elf are used to enable the MSD behaviour.
For more information run 'rpiboot -h'
On Compute Module 4 the EMMC disable jumper (GPIO 40) must be fitted to switch the ROM to usbboot mode. Otherwise, the SPI EEPROM bootloader image will be loaded instead.
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.