Fix Tinker Board S (eMMC) boot (#650) (#845)

* 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

buildroot-external/board/asus/tinker/patches/linux/board-tinkerboard-enable-emmc-support.patch

@@ -1,25 +0,0 @@
-diff --git a/arch/arm/boot/dts/rk3288-tinker.dtsi b/arch/arm/boot/dts/rk3288-tinker.dtsi
-index f88c913ff..7f04ccbdf 100644
---- a/arch/arm/boot/dts/rk3288-tinker.dtsi
-+++ b/arch/arm/boot/dts/rk3288-tinker.dtsi
-@@ -547,3 +547,20 @@
- &gpiomem {
-     status = "okay";
- };
-+
-+&emmc {
-+    bus-width = <8>;
-+    cap-mmc-highspeed;
-+    non-removable;
-+    pinctrl-names = "default";
-+    pinctrl-0 = <&emmc_clk &emmc_cmd &emmc_pwr &emmc_bus8>;
-+    max-frequency = <150000000>;
-+    mmc-hs200-1_8v;
-+    mmc-ddr-1_8v;
-+    status = "okay";
-+};
-+
-+&hdmi {
-+    pinctrl-names = "default";
-+    pinctrl-0 = <&hdmi_cec_c0>;
-+};