Fix core frequency in config.txt to repair UART console

[vstehle]

I propose that we fix the core frequency to 250 MHz to repair the UART console.

Arguably, we might be losing a bit of performance here, but the UART console is so useful for debug and headless operation that I think this is well worth it. Don't hesitate to let me know if you disagree.

Best regards, Vincent.

[bzfbd]

This helps on FreeBSD to keep serial console on a 3B+. Thank you!

[pbatard]

I don't think we want to enable that unconditionally because this will indeed result in loss of performance for people who don't care about the UART console and, ideally, it should be the OS job to provide a UART driver that handles VPU frequency changes.

What we can do then is add the following section in config.txt so that people who need this just have to uncomment the relevant line:

# Uncomment if you have trouble with the UART console during boot
#core_freq=250