Makefile: include per-target build directories in coverage report

Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>
Reviewed-by: NRichard Henderson <richard.henderson@linaro.org>

Makefile

@@ -1009,7 +1009,9 @@ $(filter %.1 %.7 %.8,$(DOCS)): scripts/texi2pod.pl
 %/coverage-report.html:
 	@mkdir -p $*
 	$(call quiet-command,\
-		gcovr -r $(SRC_PATH) --object-directory $(BUILD_DIR) \
+		gcovr -r $(SRC_PATH) \
+		$(foreach t, $(TARGET_DIRS), --object-directory $(BUILD_DIR)/$(t)) \
+		 --object-directory $(BUILD_DIR) \
 		-p --html --html-details -o $@, \
 		"GEN", "coverage-report.html")