Split configuration out from `_main.cfg`.

Split configuration macro definitions out from `_main.cfg` to a new
`config.cfg` file.
Add `NX_IS_GIT` configuration macro.

Add an `NX_IS_GIT` configuration macro, which will be defined if `.git`
exists at add-on filesystem tree toplevel.
Add `NX_GIT_HEAD` configuration macro.

Add an `NX_GIT_HEAD` configuration macro, which is defined as
`{./.git/HEAD}` if `.git` exists at add-on filesystem tree toplevel.