Ubuntu: Boot Pi from USB

grab the latest bootcode.bin, fixup.dat and start.elf and copy them to your system-boot partition.
  - https://github.com/raspberrypi/firmware

Edit the cmdline.txt file and change root=/dev/mmcblk0p2 to root=LABEL=writable

Open the config.txt Change the kernel line(s), add an initramfs line, and comment out (#) the device_tree_address line as follows:

    kernel=vmlinuz
    initramfs initrd.img followkernel
    #device_tree_address=0x03000000