The recipe creates the image for rockpro64 board bootable
with MMC or EMMC bootable devices.

New variable `sbc` (single-board computer) is introduced for selecting
the target device. This variable is needed for proper U-Boot files
selection and should have the same name as U-Boot directory targeting
to this device under `/usr/lib/u-boot/`:
 - firefly-rk3399
 - pinebook-pro-rk3399
 - puma-rk3399
 - rock-pi-4-rk3399
 - rock64-rk3328
 - rockpro64-rk3399

"rock-pi-4-rk3399" is used as a default target device.

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>