Follow-up to 5a6f829e so mkinitfs -F dhcp works as expected.
Without this change, ip=dhcp will not work on Raspberry Pi images because af_packet.ko is not in the initramfs:
$ sha256sum alpine-rpi-3.11.3-armv7.tar.gz
438648629838b02b2eead75ede2137dd480e1ae748b632b312a0b7b13924e6c2 alpine-rpi-3.11.3-armv7.tar.gz
$ for P in rpi2 rpi4; do
tar -O -xzf alpine-rpi-3.11.3-armv7.tar.gz ./boot/initramfs-$P | gunzip | cpio -t | grep "af.*ko"
done
lib/modules/5.4.12-0-rpi2/kernel/drivers/usb/storage/ums-datafab.ko
15320 blocks
lib/modules/5.4.12-0-rpi4/kernel/drivers/usb/storage/ums-datafab.ko
15552 blocks