Also switch/symlink the prebuilts and other tools. For known legacy
branches, use the -legacy suffix, -trunk else. The script is now
symlinked into the root, hence adjust the base calculation as well.

Signed-off-by: Matthias Maennich <maennich@google.com>
Change-Id: I360e48b0dc2d2eade5c648bc0eb73323c75a87ad