debian/targets.mk: Include u-boot-rockchip-spi.bin
For targets which have CONFIG_ROCKCHIP_SPI_IMAGE=y
set in the upstream
config include the image in the u-boot-rockchip package so that users
can install it to the SPI flash if they wish.
Currently debian/bin/u-boot-install-rockchip
does not attempt to
install this binary to SPI flash. It is up to users to do so.