Okay so I've moved the data folder to 'source/data' and modified BUILD_FILTER & OBJECT_FILES like so:
# TARGET SETUP #
BUILT_FILTER := $(patsubst %.v.pica,$(BUILD_DIR)/%.shbin.o,$(BUILD_FILTER)) \
$(patsubst %.shlist,$(BUILD_DIR)/%.shbin.o,$(BUILD_FILTER)) \
$(patsubst...