Please also try this post. This one is actually same as your problem.
The difference is you are using sdcard module while he is using emmc. You should append below line in “p3448-0000.conf.common” and flash:
BCFFILE=“bootloader/${target_board}/cfg/board_config_p3448.xml”;
Also, make sure board_config_p3448.xml has the correct info of your board.