Install obmc-mapper.target from obmc-targets package

The obmc-mapper.target was installed by phosphor-state-manager package.
It works fine if phosphor-state-manager is installed.

Unfortunately, x86 systems typically uses x86-power-control instead of
phosphor-state-manager to implement the state manager, and the
obmc-mapper.target is missing there. So a system with x86-power-control
does not have a valid obmc-mapper.target, and the services depdend on
the target will be started in unexpected order.

The obmc-mapper.target is really a common target, let obmc-targets.bb
install it.

Tested: Verify the g220a build has a valid obmc-mapper.target and the
        phosphor-virtual-sensor service starts after the target.
        Also verify the targets in phosphor-state-manager are not
        installed.

Signed-off-by: Lei YU <yulei.sh@bytedance.com>
Change-Id: Idb069385a6c6c7add5331a1bf3efc0babc005d62
diff --git a/meta-phosphor/recipes-core/systemd/obmc-targets/obmc-mapper.target b/meta-phosphor/recipes-core/systemd/obmc-targets/obmc-mapper.target
new file mode 100644
index 0000000..5e52f4c
--- /dev/null
+++ b/meta-phosphor/recipes-core/systemd/obmc-targets/obmc-mapper.target
@@ -0,0 +1,4 @@
+[Unit]
+Description=Phosphor Object Mapper
+RefuseManualStart=yes
+RefuseManualStop=yes