meson: move service file into its own subdir

As a part of the overall movement of phosphor-state-manager to meson,
follow the standard set in other repos and put the service files in a
subdirectory. This commit does a small refactor to the existing automake
infrastructure. Future commits will utilize this in meson.

Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
Change-Id: I63360b147d17788778c0cc8cb89e44a812ea49e8
diff --git a/configure.ac b/configure.ac
index e47c4f8..540f321 100644
--- a/configure.ac
+++ b/configure.ac
@@ -190,5 +190,5 @@
 [AC_SUBST([systemdsystemunitdir], [$with_systemdsystemunitdir])])
 
 AC_CONFIG_FILES([Makefile])
-AC_CONFIG_FILES([phosphor-systemd-target-monitor.service])
+AC_CONFIG_FILES([service_files/phosphor-systemd-target-monitor.service])
 AC_OUTPUT