poplar: save environment at a different offset

Change CONFIG_ENV_OFFSET for Poplar to be 1MB further into the eMMC
than before.  This puts it immediately prior to the space we are
reserving offset 0x200000-0x400000 for UEFI to save its persistent
data.  Define CONFIG_ENV_SIZE as a product of env_mmc_nblks and the
sector size, like CONFIG_ENV_OFFSET is.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>