Add configuration to rbm.conf to select channel and platforms

Currently we select the channel and platforms of a release with the make command we use to start the build. I think we could define this somewhere in rbm.conf, so that we can start the build with something like make torbrowser or make mullvadbrowser, automatically selecting the right channel and platforms to build.

The information about which platforms a release is for can also be used for #40994 (closed).

At the same time we can also rename var/torbrowser_version, var/torbrowser_build, var/torbrowser_incremental_from to remove the torbrowser part (since this is used in mullvadbrowser too).