rpi-base.inc: Split IMAGE_BOOT_FILES for kernel image
rpi-base.inc: Split IMAGE_BOOT_FILES for kernel image

The kernel image deployment is now defined in IMAGE_BOOT_FILES as a
separate variable: RPI_EXTRA_IMAGE_BOOT_FILES. This allows a custom
kernel image deployment by redefining this variable when needed.

Signed-off-by: Andrei Gherzan <0401b4310c02ae595fc6a2b84ead073cab9b0442@huawei.com>

agherzan/meta-raspberrypi,agherzan/meta-raspberrypi,agherzan/meta-raspberrypi,agherzan/meta-raspberrypi,agherzan/meta-raspberrypi,agherzan/meta-raspberrypi