subiquity: make subiqutiy_config.mount optional

Remove wants from local-fs.target, add wants from the uuid device instead.

Tested by applying these changes from pre-pivot-root by modifying
/root files in place whilst booting with break=bottom.

live-build/ubuntu-server/hooks/032-installer-squashfs.binary

@@ -45,6 +45,7 @@ mkdir -p "$INSTALLER_ROOT/etc/systemd/system/$DEVICE_UNIT_NAME.d"
 cat > "$INSTALLER_ROOT/etc/systemd/system/$DEVICE_UNIT_NAME.d/override.conf" <<EOF
 [Unit]
 JobRunningTimeoutSec=0s
+Wants=subiquity_config.mount
 EOF
 
 # Prepare installer layer.

live-build/ubuntu-server/includes.binary/overlay/usr/lib/systemd/system/local-fs.target.wants/subiquity_config.mount

@@ -1 +0,0 @@
-../subiquity_config.mount