image-postinst-intercepts.bbclass: add class

image-postinst-intercepts.bbclass: add class

This class sets POSTINST_INTERCEPTS and POSTINST_INTERCEPTS_CHECKSUMS,
to allow us to pull intercepts from BBPATH. This is kept as a separate
class, as it's needed by both image construction and sdk construction,
the latter to support meta-toolchain & similar recipes.

(From OE-Core rev: 7a2044df4ae8d80cf25a6bfd9b71978ffefbfa33)

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