So you have your Kali usb all setup with persistence and every time you boot from it you have to press the down arrow a few times and hit enter. It's no big deal but you can automate the process so that it boots without prompting. One way is to build a custom Kali image. That's the long way. And it's more complicated.
Here's the short way. All you have to do is change a few bytes in the iso or the device where you wrote the iso. Note that these offsets are for kali-linux-light-2016.1-i386.iso only. Remeber to replace /dev/sdX with the actual device or the iso file itself.
printf 'persistence ' | sudo dd of=/dev/sdX seek=136714379 bs=1 conv=notrunc
printf 'timeout 30\n' | sudo dd of=/dev/sdX seek=136294447 bs=1 conv=notrunc
Now you will still have 3 seconds at the grub menu in case you want to choose another option. Change the timeout to 1 and it will boot immediately (or in one-tenth of a second).
What if you don't use kali-linux-light-2016.1-i386.iso? Maybe you have one of those new computers that are 64-bit capable and your iso ends with amd64. No problem, just use a hex editor to replace all occurrences of 'hostname=kali' with 'persistence ' and 'timeout 0' with 'timeout 30\n' in the iso of your choice. Then you are all set. (It doesn't make much difference, but there should be 2 spaces after persistence.)
Try it for yourself and enjoy booting the way it was meant to be -- and without wearing out your down arrow key.
