rockchip, Makefile: add idbloader.img target

rockchip, Makefile: add idbloader.img target

Many Rockchip platforms require the same u-boot deploy procedure
when TPL and SPL both enabled.

The following examples are taken from doc/README.rockchip
and board/theobroma-systems/lion_rk3368/README:

RK3288:

  ./tools/mkimage -n rk3288 -T rksd -d ./tpl/u-boot-tpl.bin out
  cat ./spl/u-boot-spl-dtb.bin >> out
  sudo dd if=out of=/dev/mmcblk0 seek=64

RK3328:

  ./tools/mkimage -n rk3328 -T rksd -d ./tpl/u-boot-tpl.bin idbloader.img
  cat ./spl/u-boot-spl.bin >> idbloader.img
  sudo dd if=idbloader.img of=/dev/mmcblk0 seek=64

RK3368:

  ./tools/mkimage -n rk3368 -T rksd -d tpl/u-boot-tpl.bin spl-3368.img
  cat spl/u-boot-spl-dtb.bin >> spl-3368.img
  dd if=spl-3368.img of=/dev/sdb seek=64

RK3399:

  ./tools/mkimage -n rk3399 -T rksd -d ./tpl/u-boot-tpl-dtb.bin out
  cat ./spl/u-boot-spl-dtb.bin >> out
  sudo dd if=out of=/dev/sdc seek=64

Here, we introduce generic idbloader.img target
which is the TPL image followed by the SPL binary.

Signed-off-by: Matwey V. Kornilov <matwey.kornilov@gmail.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>