externalsrc: use shared stamp directory if B=S
We have a common build directory for all target architectures if
externalsrc is used and ${B}=${S}. In this case we also need to have a
common stamp directory. The reason is that e.g. changing MACHINE will
basically "invalidate" the artefacts in ${B} (wrt. to the new MACHINE)
but old stamp files with matching hashes from an earlier build might be
available in the machine-specific stamps directory and tasks would not
be correctly re-run.
Previous attempt in
f44f12af346888bdeb3ae01a275cb5dd4396b505 to handle
this correctly was faulty as it threw some spurious warnings and caused
do_configure to be re-run in every build if that would not be necessary.
[YOCTO #8950]
[YOCTO #9237]
Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
