I know (and I did it) how to make an USB key with Ubuntu 16.04 amd64 on it.
What I do need for a special hardware purpose is a kernel 4.7-rc6 on this USB key because only this will bring support for some special I2C Elan touchpad and I want to study the driver a bit.
How can I get the kernel updated to 4.7-rc6 in the USB key? Can I, for example, install a kernel from: http://kernel.ubuntu.com/~kernel-ppa/mainline/v4.7-rc6-yakkety/ after booting the key?
Btw: The USB key does not boot correctly. One has to press TAB on the boot prompt to get the input option 'live....', which seems to be a known issue, but even with this it says 'not enough memory to boot the image' (the laptop has 2 GByte of RAM) and one has to say 'live mem=2000000000' to make it booting fine.