* Fix Tinker Board S (eMMC) boot (#650)
Use Tinker Board S U-Boot configuration which is capable to boot from
eMMC as well as from SD card.
Note that this makes U-Boot always claiming to run on Tinker Board S:
..
Model: Rockchip RK3288 Asus Tinker Board S
..
It seems that there is no generic Tinker Board configuration. However,
Tinker Board S configuration really seems to work well with Tinker Board
as well, so just use it.
Also today the U-Boot Makefile seems to generate a working idbloader.img
already. Drop our special handling.
* Use Tinker Board S device tree if booting from eMMC for Linux
Instead of patching the Tinker Board device tree, select the device tree
based on what device we are booting from.
Note: This boots the non-S device tree when booting a Tinker Board S
from SD card! But there is no reliable detection otherwise, so let's
just live with that fact.
* Document how to use our U-Boot to flash eMMC
