update uboot doc

uboot-tk3308-doc/README.md

@@ -1,7 +1,9 @@
 # ~~U-boot build instructions~~ [Deprecated]
 ## U-boot is built within openwrt buildsystem
 - u-boot uses ATF blob and ROCKCHIP_TPL blob within `src/` directory.
-- SD-NAND is not tested yet.
+- ~~SD-NAND is not tested yet.~~
+- Boot from SD-NAND works with the same image as of SD card.
+
 <br>
 
 <br>
@@ -51,4 +53,4 @@ make -j12 CROSS_COMPILE=aarch64-linux-gnu- all
 ./tools/mkimage -n rk3308 -T rksd -d ..../rk3308_ddr_589MHz_uart0_m0_v2.06.bin idbloader.img 
 cat spl/u-boot-spl.bin >> idbloader.img
 cp idbloader.img u-boot.itb ../openwrt/package/boot/uboot-rockchip/src/blob/
-```
+```