Build: Export s_veneers.o

Instead of libtfm_s_veneers.a, to allow for better compatibility for
existing users of the exported files.

Change-Id: Icbcad55da56d5b0a6962138350243dd188989b97
Signed-off-by: Raef Coles <raef.coles@arm.com>
diff --git a/cmake/install.cmake b/cmake/install.cmake
index 0a1d763..1fcecc7 100644
--- a/cmake/install.cmake
+++ b/cmake/install.cmake
@@ -26,8 +26,7 @@
 
 # export veneer lib
 if (NOT TFM_MULTI_CORE_TOPOLOGY)
-    install(TARGETS tfm_s_veneers
-            ARCHIVE
+    install(FILES       ${CMAKE_BINARY_DIR}/secure_fw/s_veneers.o
             DESTINATION ${CMAKE_BINARY_DIR}/install/export/tfm/lib)
 endif()