morning, 

I want to implement a sanity check or similar which checks the if a envvar is defined in the the local.conf `FOO` but I only want throw a error if y run bitbake for a image with a particular `FEATURE`

I tried to do it with a `anonymous` function  and with `sanity check` in the meta-layer:

```
addhandler inditex_bbappend_imagecheck
inditex_bbappend_imagecheck[eventmask] = "bb.event.SanityCheck"
python inditex_bbappend_imagecheck () {
    if 'foo' in e.data.getVar('IMAGE_FEATURES').split():
        has_foo = d.getVar('FOO', True)
        if not has_foo:
            bb.fatal('FOO not defined' for image with foo feature enable)
}
```

Two problems:

* using the sanity check it seems, the check runs before and the `IMAGE_FEATURES` is not in the scope
* using the anonymous function the function is evaluated always even when I run `bitbake` for others images.

I have a workaround that is implement this in `python do_fetch_prepend () { ... }` of the package which uses the variable but this delay the error until the recipe is processed.

Any idea about how to do it during the task initialization?