ro.boot.dynamic_partitions: fix empty values

When PRODUCT_RETROFIT_DYNAMIC_PARTITIONS is empty, collapse-pairs will
write this line to build.prop:
ro.boot.dynamic_partitions=ro.boot.dynamic_partitions_retrofit=

... which is not what we want.
Test: build without dynamic partitions

Change-Id: I04c3c75fad20604657814b7f9cce2cc0635627ee
diff --git a/core/main.mk b/core/main.mk
index c323e20..d9829c5 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -244,9 +244,13 @@
 ADDITIONAL_DEFAULT_PROPERTIES += ro.actionable_compatible_property.enabled=${PRODUCT_COMPATIBLE_PROPERTY}
 endif
 
-ADDITIONAL_PRODUCT_PROPERTIES += \
-    ro.boot.dynamic_partitions=$(PRODUCT_USE_DYNAMIC_PARTITIONS) \
-    ro.boot.dynamic_partitions_retrofit=$(PRODUCT_RETROFIT_DYNAMIC_PARTITIONS)
+ifeq ($(PRODUCT_USE_DYNAMIC_PARTITIONS),true)
+ADDITIONAL_PRODUCT_PROPERTIES += ro.boot.dynamic_partitions=true
+endif
+
+ifeq ($(PRODUCT_RETROFIT_DYNAMIC_PARTITIONS),true)
+ADDITIONAL_PRODUCT_PROPERTIES += ro.boot.dynamic_partitions_retrofit=true
+endif
 
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