sdk: Add a mount unit for sysroots

Bind mount /home/sysroot on top of /opt/sysroot.
This preserves compatibility while using /home disk to
store sysroots.

Apertis: https://phabricator.apertis.org/T4391


Signed-off-by: Frédéric Dalleau <frederic.dalleau@collabora.com>

Differential Revision: https://phabricator.apertis.org/D7432

overlays/arch-sdk/lib/systemd/system/opt-sysroot.mount

+Description=Mount points for sysroots
+Before=local-fs.target
+
+[Mount]
+What=/home/sysroot
+Where=/opt/sysroot
+Type=none
+Options=rbind
+
+[Install]
+WantedBy=default.target