Here is the step-by-step to boot to U-Boot on rk3368.

Get miniloader and trust.img form rockchip vendor u-boot source code
==============================================================================
  > git clone  https://github.com/rockchip-linux/u-boot.git rockchip-uboot
  > cd rockchip-uboot
  > make rk3368_defconfig /*chose px5_defconfig if you run a px5 platform here*/
  > ./mkv8.sh

Compile the upstream U-Boot
===========================
  > cd u-boot
  > make CROSS_COMPILE=aarch64-linux-gnu- sheep-rk3368_defconfig  all

Package u-boot for miniloader
================================
  > ../rockchip-uboot/tools/loaderimage --pack --uboot u-boot.bin u-boot.img

Flash the image by rkdeveloptool
================================
rkdeveloptool can get from https://github.com/rockchip-linux/rkdeveloptool.git

Power on(or reset with RESET KEY) with MASKROM KEY preesed, and then:
  > rkdeveloptool db ./rockchip-uboot/rk3368_loader_v2.00.256.bin
  > rkdeveloptool wl 0x6000 ./rockchip-uboot/trust.img
  > rkdeveloptool wl 0x4000 ./u-boot/u-boot.img
  > rkdeveloptool RD

You should be able to get U-Boot log message from boot console:

U-Boot 2017.05-rc3-01094-g9ddd1e8-dirty (May 15 2017 - 15:57:23 +0800)

Model: Rockchip sheep board
DRAM:  2 GiB
MMC:   dwmmc@ff0f0000: 0
Using default environment

In:    serial@ff690000
Out:   serial@ff690000
Err:   serial@ff690000
Net:   Net Initialization Skipped
No ethernet found.
Hit any key to stop autoboot:  0
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