Another update:
I revised the bootmenu entry and tried again.
It should be named xxx.ext3.item because the target root partition is ext3. Also I added the fbcon module from Egoshin.

/etc/bootmenu.d/emmc.ext3.item

Code:

ITEM_NAME="EMMC"
ITEM_ID="emmc"
ITEM_DEVICE="${INT_CARD}p2" 
ITEM_MODULES="fbcon mmc_block mmc_core omap_hsmmc mbcache jbd ext3"
ITEM_FSTYPE="ext3"
ITEM_FSOPTIONS="noatime,nodiratime,commit=1,data=writeback /dev/mmcblk1p2 /mnt/new_root"

Still no success.
Then after apt-get purge bootmenu-n900 and reinstall it again, I finally saw the bootmenu. But the new rootfs still didn't boot up...