main/postmarketos-mkinitfs: use NCM instead of RNDIS (MR 3670)

As described in [0] let's switch away from RNDIS and use NCM instead.

Since we cannot force all kernels to switch at the same time, let's keep
a fallback to RNDIS in the setup function.

We can also remove usb_f_rndis from modprobe as the module gets loaded
automatically when needed, but instead we need to load libcomposite
manually so that $configfs/usb_gadget gets created.

[0] https://gitlab.com/postmarketOS/pmaports/-/issues/1797

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