Currently its hard to figure out if/when sstate is checking for possible
packages to speed up builds. This patch adds level 2 debug output which
better indicates what files are being searched for an why.
[YOCTO #1259]
(From OE-Core rev: a752b23767189f2678367e47ca0b41f49c56a631)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>