Is there a way to install IPfire on a PCengines APU4 without access to a serial console to help during the installation?
OpenWRT/OPNsense has ready made images you can flash on the SSD, but I guess if you install IPfire on a different machine and then edit the network setup somehow before moving the SSD over to the APU4 it might work?
Any other ideas how to do it?
Maybe an automatic installer like Teklager has made for OpenWRT? See: OpenWRT automatic USB installer
not that I know of…
Is there some reason you do not want to use the serial console?!?
I don’t have any suitable hardware that would allow me to use it, and for a single installation it would seem a waste to buy some.
Have you considered an inexpensive serial-to USB adapter, for connection to another PC?
It’s a convenient little thing to have. I’ve used one for connections to various access point type devices.
Furthermore, the serial port is the native console of the APU.
In case of network config problems you need the access method anyway.
To me they are fairly inexpensive:
Look, I appreciate the suggestions, but I had this APU now for 4 years and never needed a serial cable, and ordering one here is a bit complicated as it will take several weeks to arrive and cost a bunch of shipping.
I am sure there is a way to pre-configure an Ipfire installation so that I can directly connect to it via SSH or the web-interface, like it is possible with OpenWRT and OPNsense on the APU4. Help with that would be apprechiated 
To connect via network you must define a NIC as green0 ( in IPFire notation ).
IPFire installation doesn’t make those predefinition, it is designed for setup on the native console. This console is a KVM interface on most PC like systems, in case of pcengine’s APU it is a serial connection.
BTW, how do you configure the APU BIOS?
Yes, I am aware how the IPfire installation works, that is why I am asking how to work around that.
I can take the SSD out of the APU4 and install and configure IPfire on a different PC, but before inserting it back into the APU I would need to manually force re-configure the right network adapter for the APU4 so that I can afterwards reach it over the GREEN network.
I assume someone here that has IPfire already installed on their APU4 and has some deeper knowledge of the system could provide me with the required settings.
The APU4 doesn’t have a BIOS but rather runs on Coreboot and AFAIK doesn’t need any configuration there.
Coreboot is a Basic Input/Output System, usually also named firmware. And it needs sometimes configuration ( boot device ). But you are right, in most cases the settings are just usable.
The only Coreboot/BIOS config that I needed on an APU4 is to change the boot order for the drives.
I backup my IPFire APU4 box onto an external drive. And if that external drive is attached during a power down-up or a reboot, then the Coreboot/BIOS will pick the wrong drive and it will NOT boot. To keep this from happening the order is changed with Coreboot/BIOS.