I'm guessing that something is missing in my machine file, but I don't really get how to use those variables. Right now I tried to guess with:

```
DEFAULTTUNE ?= "armv7a"
TUNE_PKGARCH_tune-armv7a = "armv7a"
TUNE_PKGARCH = "${TUNE_PKGARCH_tune-${DEFAULTTUNE}}"
```