Alpine Install: from a disc to PC Engines APU

The following instructions were tested with an apu2d4 and Alpine 3.11.6.

Installing on second generation PC Engines APU systems (apu2, apu3, apu4)

1. It's probably a good idea to ensure that the BIOS / Firmware is up to date.
2. Create_a_Bootable_USB thumb drive with the current standard x86_64 image from the downloads page.
3. Attach to the serial console. Configure your terminal emulator for 115200 8n1.
4. Power on the APU.
5. When prompted, hit F10 and select to boot from USB.
6. At the boot prompt, quickly type a slash to interrupt the default boot, which has a short timeout.
7. Continue entering the remainder of this command at your leisure. It should have only one slash at the start, to be clear: /boot/vmlinuz-lts modules=loop,squashfs,sd-mod,usb-storage nomodeset console=ttyS0,115200 initrd=/boot/initramfs-lts
8. Alpine should come up as normal.
9. Proceed with installation as normal.
10. Before rebooting, edit /boot/extlinux.conf.
    1. Add SERIAL 0 115200 as the first line in the file.
    2. In the APPEND line, replace quiet with console=ttyS0,115200
11. Remove the thumb drive.
12. Cross your fingers and reboot. The system should boot properly with console on serial.
13. Edit /etc/update-extlinux.conf so that if extlinux.conf is regenerated it will retain these settings:
    1. Update the line with serial_port to read serial_port=0
    2. Update the serial_baud line to read serial_baud=115200
    3. Update the default_kernel_opts line to quiet with console=ttyS0,115200
14. Run update-extlinux
15. Examine /boot/extlinux.conf.
    1. Make sure it looks OK. Compare with /boot/extlinux.conf-old.
    2. If something looks wrong, mv /boot/extlinux.conf-old /boot/extlinux.conf to roll back the changes and try to figure out what went wrong
16. Cross your fingers and reboot. The system should boot properly with console on serial.