I think you need to take a look at this variable : MENDER_UBOOT_ENV_STORAGE_DEVICE_OFFSET
For example, you can find an example here : https://github.com/mendersoftware/meta-mender-community/blob/thud/meta-mender-atmel/templates/local.conf.append#L11
# Environment
MENDER_UBOOT_ENV_STORAGE_DEVICE_OFFSET_1 = "0x2000"
# Redundant environment
MENDER_UBOOT_ENV_STORAGE_DEVICE_OFFSET_2 = "0x4000"