Fix missing SDIMG_KERNELIMAGE variable for raspberrypi target.

Author: brocaar

layers/targets/meta-raspberrypi/conf/layer.conf

@@ -20,3 +20,10 @@ IMAGE_BOOT_FILES_remove = "uImage"
 # Configuration for the rootfs overlay.
 INITRAMFS_IMAGE_NAME = "rootfs-overlay-initramfs-${MACHINE}"
 INITRAMFS_TASK =  "rootfs-overlay-initramfs:do_image_complete"
+
+# This config option is not set for the raspberrypi machine:
+# https://github.com/agherzan/meta-raspberrypi/blob/329b50f252ed9abda98841266d126f77f3cc56a0/conf/machine/raspberrypi.conf
+#
+# Without, the following warning is raised when generating the -update images:
+# do_swuimage: BitBake variable SDIMG_KERNELIMAGE not set
+SDIMG_KERNELIMAGE_raspberrypi ?= "kernel.img"