main/postmarketos-mkinitfs: Center splash images on the screen

Alpine patches busybox to support image alignment for fbsplash in [1].

So far we don't need to align the spalshes in postmarketOS as those are generated per device. However with generic images it becomes impossible to support multiple resolutions with this approach.

Set the alignment to "Center" in fbsplash config so generic system images can reuse the splash on multiple resolutions.

[1] - https://gitlab.alpinelinux.org/alpine/aports/-/blob/master/main/busybox/0008-fbsplash-support-image-and-bar-alignment-and-positio.patch

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Alpine uses the same config here: https://gitlab.alpinelinux.org/alpine/mkinitfs/-/blob/master/initramfs-init.in#L660

"They are the same image":