Add systemd service for STM

This service does not need to be enabled in the system.
It will be started automatically if a connection to one
of the listening sockets is made. System group named stm
is required for stm-user.socket to work correctly.

Change-Id: I21276d43879ef18becb49bf3fa7694bcf842ee0a

sw/nanopi/stm-user.socket

@@ -0,0 +1,11 @@
+[Unit]
+Description=User socket activation for STM daemon
+
+[Socket]
+SocketMode=0660
+SocketGroup=stm
+ListenStream=/run/stm-user.socket
+Service=stm.service
+
+[Install]
+WantedBy=sockets.target

sw/nanopi/stm.service

@@ -0,0 +1,11 @@
+[Unit]
+Description=STM RPC
+Requires=stm-user.socket
+Requires=stm.socket
+
+[Service]
+ExecStart=/usr/bin/stm -serve
+KillSignal=SIGINT
+
+[Install]
+WantedBy=multi-user.target

sw/nanopi/stm.socket

@@ -0,0 +1,9 @@
+[Unit]
+Description=Admin socket activation for STM daemon
+
+[Socket]
+SocketMode=0660
+ListenStream=/run/stm.socket
+
+[Install]
+WantedBy=sockets.target