always generate modules when a `modules.yaml` file is provided in a recipe

[Ben Cumming]

The current workflow for building a stack with module files requires calling make twice

# build the compilers, packages, configuration and finally the module files in store
make modules
# compress the contents of store
make store.squashfs

If make store.squashfs is called directly, the module stage is skipped.

If a recipe explicitly provides a modules.yaml file, the modules should be built by default.