fw_printenv no sd/mmc support??

[nowind]

i install uboot-env package,modify /etc/fw_env.config
/dev/mmcblk0 0x88000 0x10000 0x10000
but when i run fw_printenv ,it shows
Cannot get MTD information: Invalid argument
it seem uboot-env package don't support sdcard..
i google,and fw_printenv could support sdcard ...
but why uboot-env package can not??