lib/oe/classextend: Avoid early expansion of PR values

lib/oe/classextend: Avoid early expansion of PR values

Variables like RDEPENDS can contain EXTENDPKGV which in turn uses AUTOPR
based values. This gets set during do_package execution so we want to
defer expansion until then. The only way we can do this in the RDEPENDS
(and friends) mapping code is to subsitute a dummy value, then change it
back again. Horrible but I can't see any other way.

This resolves multilib build failures with inconsistent PR values.

(From OE-Core rev: 5aea553e6eaa3b9647f26944976d2a9da79cba42)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>