flash: Clean up/add support for RPi 3

Although the official Fedora CoreOS documentation only provides
instructions for running CoreOS on a Raspberry Pi 4, it does actually
work on older boards as well.  `coreos-installer` creates a GPT disk
label, which the older devices do not support, but this can be worked
around using a hybrid MBR label.

Unfortunately, after I put all the effort into refactoring this script
and adding support for the older devices, I realized that it was rather
pointless as those boards simply do not have enough memory to be useful
Kubernetes nodes.  I was hoping to move the Zigbee and ZWave controllers
to a Raspberry Pi 3, but these processes take way too much memory for
that.

.gitignore

@@ -1,3 +1,4 @@
 *.ign
 frigate.env
 *.token
+RPi4boot