default PYTHON to python3 on linux, and python on windows

examples/make.mk

@@ -61,15 +61,16 @@ GDB = $(CROSS_COMPILE)gdb
 OBJCOPY = $(CROSS_COMPILE)objcopy
 SIZE = $(CROSS_COMPILE)size
 MKDIR = mkdir
-PYTHON = python
 
 ifeq ($(CMDEXE),1)
   CP = copy
   RM = del
+  PYTHON = python
 else
   SED = sed
   CP = cp
   RM = rm
+  PYTHON = python3
 endif
 
 #-------------- Source files and compiler flags --------------