devtool: fix extracting source for work-shared recipes

devtool: fix extracting source for work-shared recipes

Recipes that use work-shared (such as libgcc) are capable of unpacking
the source, but it doesn't necessarily unpack to ${WORKDIR}/${BP}. Use
the last part of the actual S value instead which is more likely to
work.

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