Follow-up to 5a6f829e so `mkinitfs -F dhcp` works as expected.

Without this change, `ip=dhcp` will not work on Raspberry Pi images:

    $ 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

ref aports#10750