Adding a Serial Console to a Raspberry Pi
This article describes how to add a serial console to a Raspberry Pi 3 that is running the Arch Linux distribution.
USB Serial Adapter
If simply adding a USB serial port, configure systemd as described in the article Installing Serial Terminal (AGetty).
NOTE: If using a USB Serial Port, the console output will not be seen due to order of initialisation. If using the GPIO pins, console output should be visible (see below).
GPIO Serial Port
If using the GPIO a serial port TTL to RS232 adapter will be needed and a few things need to be configured.
On a Raspberry Pi 3 the Bluetooth modem uses the ttyAMA0 port by default and, whilst there is an additional 'mini-uart' available on the GPIO, for this project it is more appropriate to disable Bluetooth and use ttyAMA0 for a serial console.
To disable Bluetooth, add the following to /boot/config.txt
Then disable the modem using systemd
systemctl disable hciuart
Continue by following the article Installing Serial Terminal (AGetty) but use ttyAMA0 as the device name.