Adjust .gitignore

Since commit cf029a87c9, I get this error:

```
$ git add --verbose kali-config/common/includes.chroot/usr/lib/live/config/0031-kali-user-setup
add 'kali-config/common/includes.chroot/usr/lib/live/config/0031-kali-user-setup'
The following paths are ignored by one of your .gitignore files:
kali-config/common/includes.chroot/usr/lib/live/config
hint: Use -f if you really want to add them.
hint: Disable this message with "git config advice.addIgnoredFile false"
```

This is because wildcards were dropped, ie `config/* became `config/`
and `chroot/*` became `chroot/`, and I don't really understand *why* it
caused the change in behavior, but let's not go down the rabbit hole.

It seems to me that we want to ignore directories at the root, so be
explicit about that, and that's enough to fix the issue.

.gitignore

@@ -1,12 +1,12 @@
 ## Directories
-.build/
-cache/
-chroot/
+/.build/
+/cache/
+/chroot/
 ## `lb` will generate this folder, `config`
 ##   Which is why our config directory is called `kali-config`
-config/
+/config/
 ## Incase symlink, no slash
-images
+/images
 
 ## Files
 .lock