Use variable for service install location

After enabling 'usrmerge', the install location for systemd service
files changed. This uses a variable to get the installation location
instead of hardcoding it.

Tested:
Confirmed that with 'usrmerge' enabled, the service file is correctly
installed.

Change-Id: Id0b5f10c7c0eb8497752fe5b46fba138cd858ec7
Signed-off-by: Jason M. Bills <jason.m.bills@intel.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 7f61aaf..c8146b0 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -42,10 +42,12 @@
 target_link_libraries(${PROJECT_NAME} ${Boost_LIBRARIES})
 target_link_libraries(${PROJECT_NAME} phosphor_logging)
 
+pkg_get_variable(SYSTEMD_SYSTEM_UNIT_DIR systemd systemdsystemunitdir)
+
 set(SERVICE_FILES
     ${PROJECT_SOURCE_DIR}/xyz.openbmc_project.U_Boot.Environment.Manager.service
     )
 
 install(TARGETS ${PROJECT_NAME} DESTINATION bin)
-install(FILES ${SERVICE_FILES} DESTINATION /lib/systemd/system/)
+install(FILES ${SERVICE_FILES} DESTINATION "${SYSTEMD_SYSTEM_UNIT_DIR}")