u-boot-imx: Create and deploy u-boot-initial-env.bin and prefilled fw_env.conf

Convert the u-boot-initial-env to an image in the u-boot env format
that can be written to disk as is.
Write the defconfig values for env size and offset to fw_env.conf
If the environment size and offset cannot be found in the u-boot
defconfig, use defaults.

BCS 746-000600