kernel-yocto: add KBUILD_DEFCONFIG search location to failure message

It was pointed out that since the check for the existence of
KBUILD_DEFCONFIG knows where it is looking, that should be part of
the error/log message.

We only look in one location for the defconfigs, so add where we
checked to the message.

Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>