Merge local changes to thermite.sh:

 - Re-implement truncate_logs() to clear log files
   when 'tail -F logs/*' is used.

 - In prebuild_setup(), recursively revert any local
   changes to the release/ checkout.

 - Remove check_x86(), since its intended use case did
   not quite match the intended implementation.

 - In main(), make the 'chroots/', 'logs/', and 'release/'
   directories on exec.

Sponsored by:	The FreeBSD Foundation