copy_buildsystem.py: Add methods to copy shared state.

Added the helper functions necessary to copy the sstate from the
current build, and generate the file to "lock" it.

Signed-off-by: Randy Witt <randy.e.witt@linux.intel.com>
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>