Serial done better

[Achllle]

Simplified instructions for serial comms setup and switch to using primary serial device (miniUART).

Before this PR, the setup caused the serial device chain to look like this:

bluetooth                   -> /dev/serial0 -> /dev/ttyS0 -> UART 1
roboclaw connection -> /dev/serial1 -> /dev/ttyAMA0      -> UART 0

which is not the recommended approach. After, it will look like:

roboclaw connection -> /dev/serial0 -> /dev/ttyS0      -> UART 1
bluetooth                   -> /dev/serial1 -> /dev/ttyAMA0 -> UART 0

When set up correctly using raspi-config, there should be no need for udev rules or /boot/firmware/config.txt hacking.

[abust005]

Changes look straightforward, I'll sign off on them

[Achllle]

Sharing my learnings from diving into getting bluetooth to work on the RPi 4.
I never got both bluetooth and the miniUART for connecting to the roboclaws to work consistently.

In fact I had to disable bluetooth altogether in the /boot/firmware/config.txt dtoverlay=disable-bt to get things to work consistently again. Adding that overlay makes the RPi use the UART 1 (PL011) instead of the miniUART for connecting to the roboclaws, which is likely going to be more consistent. I think we should add such line in so that the PL011 is used by default for everyone. That would make the mapping look like:

lrwxrwxrwx 1 root root 7 Mar 31 02:22 /dev/serial0 -> ttyAMA0
lrwxrwxrwx 1 root root 5 Mar 31 02:22 /dev/serial1 -> ttyS0

which looks like how it was before, but this time we're doing it 'the right way'.

I'm giving up on bluetooth in the short term, might try again with the RPi5 since that doesn't have the miniUART anymore.

It would be good for someone to give this PR a test just to make sure this works fine. I need to add instructions for adding the disable bluetooth overlay.