bitbake: runqueue.py: Fix a stamp comparision bug

bitbake: runqueue.py: Fix a stamp comparision bug

We check the stamp cache before comparing t2 and t3 which means that we can miss
a level in the stamp file chains. The result of this is that a stamp can be accepted
as valid when in fact it isn't. Some weird behaviour alerted me to this in a local
build.

This patch also fixes to only uses the cache in recurse mode, there was a corner
case where stamps not in recurse mode could get added to the cache which could cause
an issue potentially.

(Bitbake rev: 7a5f776d325c0e37f58003424beda7cae2695325)

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