editorconfig: add exceptions for Makefiles

- [*.mk]
- [Makefile]

OpenWrt and Gluon use spaces for a better overview,
tabs are used for `make` to mark recipes as such.

.editorconfig

@@ -35,6 +35,9 @@ indent_size = 2
 
 [*.lua]
 
+[{Makefile,*.mk}]
+indent_style = unset
+
 [*.md]
 indent_style = space
 indent_size = 4