[script] Now top-level makefile a command 'format-py' to apply python formatting

Makefile

@@ -43,6 +43,7 @@ endif
 PYTHON_EXEC ?= python3
 CLANG_FORMAT_EXEC ?= clang-format-10
 XML_FORMAT_EXEC ?= xmllint
+PYTHON_FORMAT_EXEC ?= black
 
 # Put it first so that "make" without argument is like "make help".
 export COMMENT_EXTRACT
@@ -83,6 +84,13 @@ format-xml:
 	XMLLINT_INDENT="  " && ${XML_FORMAT_EXEC} --format $${f} --output $${f} || exit 1; \
 	done
 
+format-py:
+# Format all the python scripts under this project, excluding submodules
+	for f in `find openfpga_flow/scripts -iname *.py`; \
+	do \
+	${PYTHON_FORMAT_EXEC} $${f} --line-length 100 || exit 1; \
+	done
+
 clean:
 # Remove current build results
 	rm -rf ${BUILD_DIR} yosys/install