base.bbclass: Fix PACKAGECONFIG issues with native and nativesdk BBCLASSEXTEND recipes (and multilib)
This patch fixes up the issues that were being seen where BBCLASSEXTEND and
PACKAGECONFIG were interacting badly. It also ensures PACKAGECONFIG interacts
properly with multilib builds.
Ideally some of this code will be abstracted into lib/oe/classextend.py but
at this point in release more invasive changes like this are inappropriate.
This patch also removed empty strings from expressions rather than
passing them around as this was complicating the additional code
unnecessarily.
The patch was verified against the OE-Core metadata where the return values of
expandFilter() were sanity checked by hand for native/nativesdk and
multilib combinations.
[YOCTO #2225]
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
