The sdcard class in the meta-raspberrypi layer is now extensible with
support for adding files such as hypervisor binaries to the boot
partition, so this layer can switch to use that and drop the custom
sdcard bbclass.

Uses the new RPI_SDIMG_EXTRA_DEPENDS and DEPLOYPAYLOAD variables,
and: IMAGE_CLASS: sdcard_image-rpi-xen => sdcard_image-rpi
    IMAGE_FSTYPE: rpi-xen-sdimg => rpi-sdimg

Please note that the filename suffix of the image is changed by this.

Also apply _raspberrypi4-64 scoping to the Raspberry Pi settings.

Signed-off-by: Christopher Clark <christopher.w.clark@gmail.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>