tools: fix toolchain extend inc paths

inc paths might be a list or might not be (just single string). If they don't, we are ending up with non valid include paths (one letter include paths).
This as result would not compile.

tools/toolchains/__init__.py

@@ -863,7 +863,10 @@ class mbedToolchain:
 
         inc_paths = resources.inc_dirs
         if inc_dirs is not None:
-            inc_paths.extend(inc_dirs)
+            if isinstance(inc_dirs, list):
+                inc_paths.extend(inc_dirs)
+            else:
+                inc_paths.append(inc_dirs)
         # De-duplicate include paths
         inc_paths = set(inc_paths)
         # Sort include paths for consistency