refactor(mediatek): refactor handling of variables with value 'n'

Refactor to properly handle the case when the variable has a value
of 'n'.

Change-Id: I2f5045253511ec5e5d717821d8428c4e3ed6b7b6
diff --git a/plat/mediatek/build_helpers/mtk_build_helpers.mk b/plat/mediatek/build_helpers/mtk_build_helpers.mk
index 87a7db4..a497049 100644
--- a/plat/mediatek/build_helpers/mtk_build_helpers.mk
+++ b/plat/mediatek/build_helpers/mtk_build_helpers.mk
@@ -29,7 +29,7 @@
 ifdef $(1)
 ifeq ($(strip $(value $(1))),y)
 DEFINES += -D$(1)$(if $(value $(1)),=1,)
-else
+else ifneq ($(strip $(value $(1))),n)
 DEFINES += -D$(1)$(if $(value $(1)),=$(value $(1)),)
 endif
 endif