create-spdx: default share_src for shared sources

if a source is using work-shared but isn't a kernel,
like for instance llvm-source from meta-clang, share_src was
previously undefined leading to a crash of the python code.
Default to WORKDIR and just override it in case the source being
a kernel recipe.
Additionally changes the variable names in the following, as
they imply that it's only about the kernel, which is not the case
in every case

Signed-off-by: Konrad Weihmann <kweihmann@witekio.com>
On-behalf-of: Avnet Embedded <AvnetEmbedded@avnet.eu>
Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Reviewed-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>