Make dev-check check for SSH instead of non-HTTP

In order to determine, whether we are developers or not, check the git-remotes for SSH-URLs instead of checking for the non-existence of any HTTP-URLs. This fixes dev-mode not being detected properly when curious devs add other HTTP-remotes such as upstream OpenWRT.

This also makes the check consistent with the check done in the main "iop" script in the root.

Random comment:

Ideally we should have one central place for this check and in my opinion there should be an easy way to override the mode permanently (i.e. outside of .config). The current detection is okay for the default-setting, but checking the configured git-remotes is too much magic for such a setting in my opinion.