Activation: initially support psu update

Initial support for PSU update by starting a systemd unit with PSU
inventory path and image dir as arguments.

Add an example psu-update@.service that shows how the arguments are
passed to systemd unit and expanded to command line arguments.

Tested: Upload a dummy tarball, create a dummy service that only prints
        the arguments, and verify the service is invoked correctly when
        the RequestedActivation is set to Active.

Signed-off-by: Lei YU <mine260309@gmail.com>
Change-Id: I7e122f1cce234caf4951d3e3daad5bee406b507b
diff --git a/vendor-example/psu-update@.service b/vendor-example/psu-update@.service
new file mode 100644
index 0000000..06635be
--- /dev/null
+++ b/vendor-example/psu-update@.service
@@ -0,0 +1,17 @@
+# This service shall be started with two arguments:
+#  * The PSU inventory DBus object
+#  * The path of the PSU images
+# E.g.
+#  "psu-update@-xyz-openbmc_project-inventory-system-chassis-motherboard-powersupply0\x20-tmp-image-abcdefg.service"
+# expands to
+#  /usr/bin/psutils --update /xyz/openbmc_project/inventory/system/chassis/motherboard/powersupply0 /tmp/image/abcdefg
+
+[Unit]
+Description=Update PSU %I
+
+[Service]
+Type=oneshot
+RemainAfterExit=no
+Environment="ARGS=%I"
+ExecStart=/usr/bin/psutils --update $ARGS
+SyslogIdentifier=psutils