core/Makefile: pass value of PRODUCT_USE_DYNAMIC_PARTITION_SIZE

Regardless of the value of PRODUCT_USE_DYNAMIC_PARTITION_SIZE
"use_dynamic_partition_size=true" is passed to build_image.sh.
Pass the value of PRODUCT_USE_DYNAMIC_PARTITION_SIZE just like it is
done for other properties.

Test: 1. build a target with fixed partition sizes and
         "PRODUCT_USE_DYNAMIC_PARTITION_SIZE := false".
      2. check the value of "use_dynamic_partition_size" in the
         corresponding system_image_info.txt.
      3. repeat steps 1 and 2 with this fix and see the difference.

Change-Id: Ia121d3d67f554f8ae4e211ad4e2c381a43409f01
diff --git a/core/Makefile b/core/Makefile
index c862e97..af03c2e 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -1714,7 +1714,8 @@
 $(if $(filter true,$(BOARD_BUILD_SYSTEM_ROOT_IMAGE)),\
     $(hide) echo "system_root_image=true" >> $(1))
 $(hide) echo "root_dir=$(TARGET_ROOT_OUT)" >> $(1)
-$(if $(PRODUCT_USE_DYNAMIC_PARTITION_SIZE),$(hide) echo "use_dynamic_partition_size=true" >> $(1))
+$(if $(filter true,$(PRODUCT_USE_DYNAMIC_PARTITION_SIZE)),\
+    $(hide) echo "use_dynamic_partition_size=true" >> $(1))
 $(if $(3),$(hide) $(foreach kv,$(3),echo "$(kv)" >> $(1);))
 endef