Fixing missing import for os.path.exists

Author: bridadan

tools/toolchains/iar.py

@@ -16,7 +16,7 @@
 """
 import re
 from os import remove
-from os.path import join, splitext
+from os.path import join, splitext, exists
 
 from tools.toolchains import mbedToolchain, TOOLCHAIN_PATHS
 from tools.hooks import hook_tool