How to enable the uart log in user version

firstly:
To find the string DEBUG_KERNEL_CMDLINE
grep DEBUG_KERNEL_CMDLINE in yourproject root directory.
maybe in yourprojector/device/mtk/BoardConfig.mk file. There is the DEBUG_KERNEL_CMDLINE string.

secondly:
do as the eng version to modify the DEBUG_KERNEL_CMDLINE 's string
such as
DEBUG_KERNEL_CMDLINE:= loglevel=0
modify to
DEBUG_KERNEL_CMDLINE:= console=ttyS0,115200, loglevel=8

thirdly:
make bootimage

finnally:
flash the the new boot.img

fastboot flash boot boot.img

By the way:
if you have the only boot.img
try to use the binary editor to add the ' console=ttyS0,115200,loglevel=8 ' string.
payattention: you must manually type the above string, should not copyand paste
then save and fastboot flash bootboot_modify.img
after do that, you can get the kernel log as the eng version in uart port.