fix(build): mute sp_mk_generator from build log

Add a silent token to sp_mk_generator python script invocation.

Signed-off-by: Olivier Deprez <olivier.deprez@arm.com>
Change-Id: I51d9bd6965504d8869e946db0a74691bbaa1dcd2
diff --git a/Makefile b/Makefile
index 3536763..ac73816 100644
--- a/Makefile
+++ b/Makefile
@@ -1585,8 +1585,8 @@
 
 # Add Secure Partition packages
 ifeq (${NEED_SP_PKG},yes)
-$(BUILD_PLAT)/sp_gen.mk : ${SP_MK_GEN} ${SP_LAYOUT_FILE} | ${BUILD_PLAT}
-	${PYTHON} "$<" "$@" $(filter-out $<,$^) $(BUILD_PLAT) ${COT} ${SP_DTS_LIST_FRAGMENT}
+$(BUILD_PLAT)/sp_gen.mk: ${SP_MK_GEN} ${SP_LAYOUT_FILE} | ${BUILD_PLAT}
+	@${PYTHON} "$<" "$@" $(filter-out $<,$^) $(BUILD_PLAT) ${COT} ${SP_DTS_LIST_FRAGMENT}
 sp: $(DTBS) $(BUILD_PLAT)/sp_gen.mk $(SP_PKGS)
 	@${ECHO_BLANK_LINE}
 	@echo "Built SP Images successfully"