Makefile: Add QMK_VERSION & co to OPT_DEFS (#408)

Makefile: Add QMK_VERSION & co to OPT_DEFS (#408)

This adds the keyboard and keymap built, along with the QMK firmware's
git hash (or a timestamp), to OPT_DEFS. That, in turn, allows keymaps to
make use of these information, and do whatever they want with it. For
example, one could print them on `LEADER v` like this:

```c
SEQ_ONE_KEY (KC_V) {
  SEND_STRING (QMK_KEYBOARD "/" QMK_KEYMAP " @ " QMK_VERSION);
}
```

This addresses #366.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>