Git fix for empty directory. Makefile updated.

- ‘build’ directory was not being synced by git because it was empty.
Added a .gitignore file in the ‘build’ directory to force git to sync
it but keep it empty.

- Updated the Makefile to not erase the .gitignore.

Makefile

@@ -77,7 +77,7 @@ load: all
 	bootloadHID grbl.hex
 
 clean:
-	rm -f grbl.hex $(BUILDDIR)/*
+	rm -f grbl.hex $(BUILDDIR)/*.o $(BUILDDIR)/*.d $(BUILDDIR)/*.elf
 
 # file targets:
 $(BUILDDIR)/main.elf: $(OBJECTS)

build/.gitignore

@@ -0,0 +1,4 @@
+# Ignore everything in this directory
+*
+# Except this file
+!.gitignore