Add support for generation of uboot.env image from txt file

Fixes: https://github.com/OP-TEE/build/issues/85


Signed-off-by: Igor Opaniuk <igor.opaniuk@linaro.org>
Tested-by: Igor Opaniuk <igor.opaniuk@linaro.org> (RPi3)
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>