package bbclass: allow per package PRIVATE_LIBS

package bbclass: allow per package PRIVATE_LIBS

If a recipe packages multiple versions of shlib (e.g. powervr drivers) we only want the shlib code to pickup $PN, not $PN-foo subpackages.
This keeps backward compatibility with the global PRIVATE_LIBS usage if no per package PRIVATE_LIBS are set for a given package. In other words: this doesn't break the firefox recipe.

(From OE-Core rev: ce46dbddef40ae3eef7238ac07438b15bd09e156)

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>