sdk: Enable the /opt/sysroot → /home/sysroot mount unit

Actually enable the opt-sysroot.mount unit to point `/opt/sysroot` to
`/home/sysroot` in order to not run out of space on the root fs.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>

overlays/sdk-mount-opt-sysroot/lib/systemd/system/default.target.wants/opt-sysroot.mount

+../opt-sysroot.mount
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