[Git] Use https or relative path for submodule urls

[MartinHerren]

Hello,

Wanting to try out transmitter-linux i cloned recursively this repo and the cloning failed on the sub modules.

The issue is that i used a computer of which i didn't authorize the ssh keys on github.

The simple fix is of course to add the public key to github.

As this is a public repo it would make sense to be able to clone it without a github account. So i replaced the sub module urls with https urls instead of ssh ones.

For the opendroneid-core-c module, as it is in the same repo, i used a relative path, this way

• if someone clones transmitter-linux using https, opendroneid-core-c will be cloned using https as well
• if someone clones using ssh, the sub module will be cloned using ssh as well
• in case one day you'd move the whole opendroneid project to some other git server, the relative url wouldn't change

The hostapd and bluez urls work with anonymous ssh, so i could revert this part of the PR if you prefer.

Thanks for this nice project !

[friissoren]

Thanks. This makes sense. Consistency is always a good thing.