Here I am going to show you how to get the boot.img (kernel) out of an FTF! This procedure is most useful for Locked Bootloaders as it means we can upgrade without the need for FlashTool!
This guide assumes that you have FlashTool installed on your PC
You need a file extractor such as WinRar or 7zip.
Once this is done you want to open the whole FTF in the extractor. You will see a lot of .sin files!
The one we want is 'kernel.sin', not 'fotakernel.sin'.
Extract that file to somewhere appropriate on your computer, like the desktop!
Now you need to open FlashTool,
At the top you will see a tab that says 'Tools', click on that and select 'SIN Editor'. You will see a button that has 3 dots in the top right. This is the browse button, click on it and locate where your SIN file is.
One that is done, you need to click on 'Dump Data'.
This will dump the 'kernel.elf' to the same location of where your 'kernel.sin' is.
The next part is simple,
All you need to do is rename your 'kernel.elf' to 'boot.img' and put it in a flashable zip, to make the flashable zip follow the instructions by @DaRk-L0rD in the 2nd post!
Keep in mind that the kernel should be 20mb.
Enjoy! The kernel can be flashed via PhilZ for Locked Bootloaders and will work on both Locked and Unlocked bootloaders, since it is an official kernel it is signed by the Sony key so will be fine for Locked Bootloaders.