main/postmarketos-mkinitfs: include ext4 and usb_f_rndis in initramfs (!673)

The initramfs needs ext4 (to mount the rootfs) and usb_f_rndis
(to enable USB networking on devices that use USB configfs).
If these are built as modules, they need to be included in initramfs.

[ci:skip-build]: already built successfully in CI