sstate.bbclass: specify func dirs for sstate_hardcode_path

sstate.bbclass: specify func dirs for sstate_hardcode_path

For some recipes that inhrient cmake, the ${B} may be removed by
cmake_do_configure() while sstate_hardcode_path() running, this
causes build errors:

  Exception: OSError: [Errno 2] No such file or directory: \
  '/path/to/build'

The function sstate_hardcode_path() called command:

  $SSTATE_SCAN_CMD which extended as "find ${SSTATE_BUILDDIR} ..."

So the proper function dirs could be ${SSTATE_BUILDDIR}.

Signed-off-by: Wenzong Fan <wenzong.fan@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>