buildmaster: entry.sh: fix errors by checking for dir existence

buildmaster: entry.sh: fix errors by checking for dir existence

Get rid of following annyoing errors during testing:

  chown: cannot access '/config': No such file or directory
  chown: cannot access '/certs': No such file or directory
  chmod: cannot access '/config': No such file or directory
  chmod: cannot access '/certs': No such file or directory

As those dirs doesn't exist in the container, they're being provided as
volumes during deployment.

Signed-off-by: Petr Štetiar <[email protected]>