This is the first alpha of the 12.5 series.

This commit adds a config variable to override the URL for the updater.
When it is defined, a user-provided certificate will replace the
default public keys used to sign nightly builds.

Add the needed targets needed to build privacy browser.

With #23403 we started using the tor-browser-build/tmp directory for the
builds.

Add a leading slash on all lines to match files/directories only in the
root directory.

Only for linux-x86_64 for now, as it fails to build on linux-i686.

To provide a more "high level" view of progress, don't output build logs
of each componets on stdout, but write them in files in the logs/ directory.

This idea was suggested by mcs and Kathy in this comment:
https://trac.torproject.org/projects/tor/ticket/17379#comment:12

Update rbm and add example rbm.local.conf file.