Add definition for new platform in rbm.conf

In order to be able to integrate Tor Browser for Android into tor-browser-build, we need to add a definition for the new platform into rbm.conf.

To do that we need to decide:

which OS we want to use inside the build containers. Probably Debian Stable, unless we have a reason for using something else.
how we want to name this new tor browser platform in tor-browser-build/rbm.conf. Currently we have: linux-x86_64, linux-i686, windows-i686, windows-x86_64 and osx-x86_64.

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