These instruction should help you in flashing the bootloader
and AOSP images built from sources.
Detailed instructions on how to setup the Board can be found
here:
https://github.com/96boards/documentation/wiki/HiKeyGettingStarted

For flashing the bootloader, the top two links should be installed
(closed) and the 3rd link should be removed (open):

---------------------------------------------------------
|	Name 		|	Link 	|	State 	|
| Auto Power up 	|     Link 1-2 	|      closed 	|
| Boot Select 		|     Link 3-4 	|      closed 	|
| GPIO3-1      		|     Link 5-6 	| 	open 	|
---------------------------------------------------------

Link 1-2 causes HiKey to auto-power up when power is installed. Link
3-4 causes the HiKey SoC internal ROM to start up in at a special
"install bootloader" mode which will install a supplied bootloader
from the microUSB OTG port into RAM, and will present itself to a
connected PC as a ttyUSB device.

Note: The HiKey board will remain in USB load mode for 90 seconds
from power up. If you take longer than 90 seconds to start the install
then power cycle the board before trying again.

Wait about 5 seconds and then check that the HiKey board has been
recognized by your Linux PC:
$ ls /dev/ttyUSB*
or
$ dmesg


Run the flash-all.sh script after building AOSP for hikey with the right
right UART recognised in the above command:
$ ./flash-all.sh /dev/ttyUSBX [4g]

Remove the link 3-4 and power on the board.