Makefile options for `oldestable` images

Thank you for the great work on this repo. 

I would like to suggest adding options for building the image with the old images (`bullseyes`).  I was able to do it by modifying the  Makefile entry for my board and replacing:
```raspios=raspios_lite_armhf ```
with
```raspios=raspios_oldstable_lite_armhf```