If you are using 15.06 you wont be seeing anything on the serial port by default (I honestly dont know what it was decided to make the release that way but that is the way it is)
To fix this just mount the boot_fat.img partition and modify the cmdline so it looks something like this.
k3v2mem hisi_dma_print=0 vmalloc=484M no_irq_affinity ignore_loglevel earlyprintk console=ttyAMA3,115200n8 androidboot.console=ttyAMA3 androidboot.hardware=hikey root=/dev/mmcblk0p7 firmware_class.path=/system/etc/firmware
1. sudo mount boot_fat.img <mnt_point>
2. sudo vi <mnt_point>/cmdline
Then edit/replace the contents of the file with the above.