Revise busybox i2ctools make

For Issue #61
Since the config is optional you should add it to the dependencies for $(BUSYBOX_DIR)/.config.

Just like you test and set MACHINE_BUSYBOX_PATCHDIR_FILES above you should do something similar for the optional config file.  Something like:

```bash
MACHINE_BUSYBOX_CONFIG_FILE = $(shell \
			   test -f $(MACHINE_BUSYBOX_CONFDIR)/config && \
			   echo "$(MACHINE_BUSYBOX_CONFDIR)/config")
```

Then add $(MACHINE_BUSYBOX_CONFIG_FILE) to the dependencies for $(BUSYBOX_DIR)/.config like this:

```makefile
$(BUSYBOX_DIR)/.config: $(BUSYBOX_CONFIG) $(MACHINE_BUSYBOX_CONFIG_FILE) $(BUSYBOX_PATCH_STAMP)
```

This will help the case during development when you update your machine's busybox config and you want to do an incremental build.  Otherwise $(BUSYBOX_DIR)/.config will not be out of date.