Partial compatibility for on-device builds.
There is no guarantee that it will be possible to build all available
packages and built packages will have same reliability that cross-compiled
but should solve "self-hosting" problems as much as possible.
Can be set to the path to packages/ directories in other repos (like
unstable-package/packages/, termux-root-packages/packages/,
..). buildorder.py then searches these directories for packages and
dependencies