configs/raspberrypi0w: fix dts file name after kernel bump

configs/raspberrypi0w: fix dts file name after kernel bump

Fixes #12816

Commit 42d22f3bdba41da ({configs/raspberrypi,package/rpi-firmware}: bump
kernel/firmware to 20190819 version) updated the kernel version but failed
to take into consideration that the rpi0w dts file has been renamed:

https://github.com/raspberrypi/linux/commit/6f91b5dbfdb62a434571a73f2dc15181963e3bea

Fix it by renaming the dts/dtb file referenced from the kernel build and
genimage.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>