oe.package_manager: support loading intercepts from multiple paths

oe.package_manager: support loading intercepts from multiple paths

- if POSTINST_INTERCEPTS is set, use the listed intercept files, or
- if POSTINST_INTERCEPTS_PATH is set, load from the listed paths, or
- if POSTINST_INTERCEPTS_DIR is set, load from it (for compatibility), or
- load from ${COREBASE}/meta/postinst-intercepts

(From OE-Core rev: 9ba2f2b1df277b2b881f68166d9cd1c19db66e23)

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>