kernel-yocto: fix kernel-meta data detection

Fixes: 7ef7af5c ("kernel-yocto: restore kernel-meta data detection for SRC_URI elements")
Signed-off-by: Ulrich Ölmann <u.oelmann@pengutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>