The nameing of the bootfiles have been historically inconsistent. To
fix that here is the convention to be used for all bootfiles after
2014-07.

1. The bootfile must reside in the directory whose name is the current
   CMUCL version for which this bootfile applies.  For example, as of
   this writing, the current version is 20e, so boot files must be in
   the 20e directory.
2. The name of the boot file shall be boot-yyyy-mm-n.lisp where
   yyyy-mm represents the year (yyyy) and month (mm) of the snapshot
   that is used to build from.  The boot files are numbered
   sequentially from 1 (n).  Thus, the boot-2014-06-1.lisp is intended
   to be built from the 2014-06 snapshot, and is the first boot file
   to be applied.
