kernel-yocto.bbclass: say what SRC_URI entry is being dropped

kernel-yocto.bbclass: say what SRC_URI entry is being dropped

If there is a defconfig set by KBUILD_DEFCONFIG and something that looks
like a defconfig in SRC_URI, the SRC_URI is dropped. This happens even
if the entry in SRC_URI is called my_defconfig_frag.cfg.

Arguably, this is a bug and the behaviour should be changed, but for now
if we're going to remove entries from SRC_URI we should make it clear
what entry is being removed.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>