Ensure EXTRA_OPTS is checked before adding it to entrypoint

[dsimog01]

This issue is tightly related to dappnode/DAppNode#549

During the building process of each package. the SDK should perform the following tasks for each service that includes the EXTRA_OPTS flag:

1. Check that the service also includes the FLAGS env (ideally, we should check that these flags are not present in the entrypoint.sh, but this might be more complex).
2. Check that the package includes check_extra_opts.sh and that it follows the exact expected format
3. Check that the last line of the entrypoint.sh follows the format `./command check_extra_opts(FLAGS, EXTRA_OPTS)

[pablomendezroyo]

Hard to implement