Implement simple terminal

The terminal uses console.Vga as its output device. A proper terminal
implementation would be using a console.Console interface as its output.
However, at this point we cannot use Go interfaces as the fn pointers in
the itables have not been yet initialized. The Go runtime bits that set
up the itables need access to a memory allocator, a facility which is
not yet provided by the kernel.

kernel/driver/video/console/vga_test.go

@@ -202,3 +202,15 @@ func TestVgaWrite(t *testing.T) {
 		t.Errorf("expected call to Write() to set fb[0] to %d; got %d", expVal, got)
 	}
 }
+
+func TestVgaOverrideFb(t *testing.T) {
+	var cons = Vga{}
+	cons.Init()
+
+	fb := []uint16{}
+	cons.OverrideFb(fb)
+
+	if len(cons.fb) != len(fb) {
+		t.Fatalf("expected calling OverrideFb to change the framebuffer for the console")
+	}
+}