Which baud rate did you choose ? All RockChip SoC are defaulting to 1500000 baud ...   EDIT : Sorry, I didn't look at your screenshot right ... So, maybe the eMMC has Android installed by default and choose a different baud rate. This Android U-Boot maybe doesn't provide option to stop and boot from eMMC.   EDIT2: To boot from SDCard, you will have to find TP50265 and TP50272 test points and short them together, that will disable the eMMC.