Updated kernel to 4.19.83

This kernel includes a number of fixes to USB and other drivers -- if you were having trouble with USB v22 is worth a try for you!

Fixed issue where lightdm service (used by xubuntu-desktop) would not load properly

Added bootcode.bin to /boot/firmware to allow older Pis to boot the image (3B+ confirmed to work so far, others need testing)

Added README documentation to /boot/firmware/overlays folder for documentation inside dtoverlay command

Updated firmware