Build: Add missed dependency to scatter files

On isolation level 2 and 3 scatter files are made from templates
processed together with manifests. The missed dependency cuased
occasional build failures.

Signed-off-by: Anton Komlev <anton.komlev@arm.com>
Change-Id: Ib4efec643d32c5df6644cce15694f3ff2c175f59
diff --git a/toolchain_GNUARM.cmake b/toolchain_GNUARM.cmake
index 99f2324..200caeb 100644
--- a/toolchain_GNUARM.cmake
+++ b/toolchain_GNUARM.cmake
@@ -277,6 +277,11 @@
         PRIVATE
             $<$<NOT:$<BOOL:${CONFIG_GNU_LINKER_READONLY_ATTRIBUTE}>>:READONLY=>
     )
+
+    # Scatter file shall be preprocessed by manifest tool in isolation level 2,3
+    add_dependencies(${target}_scatter
+        manifest_tool
+    )
 endmacro()
 
 macro(add_convert_to_bin_target target)