add incrementation migration logic (#4)

build/sysroot/etc/casaos/user-service.conf.sample

@@ -0,0 +1,9 @@
+[common]
+RuntimePath=/var/run/casaos
+
+[app]
+LogPath = /var/log/casaos
+LogSaveName = user-service
+LogFileExt = log
+DBPath     = /var/lib/casaos
+UserDataPath = /var/lib/casaos

build/sysroot/usr/lib/systemd/system/casaos-user-service.service

@@ -0,0 +1,12 @@
+[Unit]
+After=casaos-gateway.service
+ConditionFileNotEmpty=/etc/casaos/user-service.conf
+Description=CasaOS User Service
+
+[Service]
+ExecStart=/usr/bin/casaos-user-service -c /etc/casaos/user-service.conf
+PIDFile=/var/run/casaos/user-service.pid
+Restart=always
+
+[Install]
+WantedBy=multi-user.target