lib/bb/providers: make "checking PREFERRED_PROVIDER_%s" a debug message

lib/bb/providers: make "checking PREFERRED_PROVIDER_%s" a debug message

In verbose mode there are hundreds of these "checking PREFERRED_PROVIDER_%s"
messages, cluttering the output and obscuring the more important resulting
"selecting %s to satisfy runtime %s due to %s" messages. Individual "checking"
lines are more suited for debug mode, similar to "sorted providers for %s
are: %s", hence convert logger.verbose() to logger.debug().

(Bitbake rev: 85dfbec26abb5b944758f5c4749b7df16c0fb2e6)

Signed-off-by: Denys Dmytriyenko <denys@ti.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>