This kernel is configured according to JohnT's instructions
at https://gnipsel.com/linuxcnc/uspace/rpi4-rt.html and then built
with "make deb-pkg".

It only works on PI4s

After installing the package, put the following content in
/boot/config-rt.txt:

    [pi4]
    kernel=vmlinuz-4.19.71-rt24-v7l+

And add this content in /boot/config.txt:

    [all]
    include config-rt.txt

Now, you can enable/disable the RT kernel by un/commenting the line
in /boot/config.txt .

You *DO* have to have a matching upstream kernel installed, because
the devicetree files in /boot are from there, not from the rt
kernel.