host-graceful-quiesce: introduce new target

Per design here:
 https://gerrit.openbmc-project.xyz/c/openbmc/docs/+/52874

This introduces a new target which can be started to gracefully quiesce
the host. This allows the host an opportunity to shut itself down
gracefully.

Tested:
- Built image with new target and included pldmSoftPowerOff.service in
  it so that host firmware got a chance to gracefully shut down prior to
  Quiesce.

Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
Change-Id: I1b5d609b50951e5d529379196c36c0d66e429a8f
diff --git a/target_files/obmc-host-graceful-quiesce@.target b/target_files/obmc-host-graceful-quiesce@.target
new file mode 100644
index 0000000..6f2b8b5
--- /dev/null
+++ b/target_files/obmc-host-graceful-quiesce@.target
@@ -0,0 +1,4 @@
+[Unit]
+Description=Graceful Quiesce Target
+After=multi-user.target
+Conflicts=obmc-chassis-poweroff@%i.target