First run command "dd if=/dev/zero of=/dev/mmcblk0 bs=512 seek=1536 count=16" to clean up u-boot parameter in SD/eMMC card. If there is no boot parameter in SD/eMMC card, the default boot parameter will be used.
Then modify u-boot-2009.08/include/configs/mx6dl_sabresd.h(depends which platform you use).
#define CONFIG_EXTRA_ENV_SETTINGS \
"netdev=eth0\0" \
"ethprime=FEC0\0" \
"uboot=u-boot.bin\0" \
"kernel=uImage\0" \
"nfsroot=/opt/eldk/arm\0" \
"bootargs_base=setenv bootargs console=ttymxc0,115200\0"\
"bootargs_nfs=setenv bootargs ${bootargs} root=/dev/nfs "\
"ip=dhcp nfsroot=${serverip}:${nfsroot},v3,tcp\0"\
"bootcmd_net=run bootargs_base bootargs_nfs; " \
"tftpboot ${loadaddr} ${kernel}; bootm\0" \
"bootargs_mmc=setenv bootargs ${bootargs} ip=dhcp " \
"root=/dev/mmcblk0p1 rootwait\0" \
"bootcmd_mmc=run bootargs_base bootargs_mmc; " \
"mmc dev 3; " \
"mmc read ${loadaddr} 0x800 0x2000; bootm\0" \
"bootcmd=run bootcmd_net\0" \
Grace