Current situation
Raspberry Pi 4 doesn't have onboard NIC support.

Impact
After chaining off a matchbox configuration flatcar-linux can't reach it's matchbox endpoint after booting into flatcar-linux-installer

See photo

Ideal future situation
Enable the Raspberry Pi 4 networking stack for better Arm64 support.

Implementation options
I'm not a kernel expert but I believe https://github.com/flatcar-linux/coreos-overlay/blob/e4c239ab96ec750717978351f5db6494934d0cec/sys-kernel/coreos-modules/files/commonconfig-5.9#L78 should be Y instead of M.

CONFIG_BROADCOM_PHY=m

The Raspberry Pi 4 interface has a BCM54213PE as noted here on page 11.

According to kernel documentation enabling this option enables support for BCM5421

Additional information

These Raspberry Pis are running UEFI with iPXE to chainload matchbox configured with the Typhoon Kubernetes distribution. After chain loading the latest alpha build if I plug in a usb NIC where the error shows connection refused the installer continues to load but doesn't proceed since the MAC address has changed. Before opening a PR to enable CONFIG_BROADCOM_PHY=Y I wanted to hear some input on this.