sstate: add support for caching shared workdir tasks

sstate: add support for caching shared workdir tasks

The sstate bbclass uses workdir as a hardcoded string in path
manipulations. This means that the sstate caching mechanism does
not work for the work-shared directory which the kernel uses to
share its build configuration and source files for out of tree
kernel modules.

This commit modifies the path manipulation mechanism to use the
work-shared directory if detected in the paths when handling the
sstate cache packages.

(From OE-Core rev: 27642449f95e38598f9c83948ce109c5891e5877)

Signed-off-by: Michael Ho <Michael.Ho@bmw.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>