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>

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Igor Opaniuk <igor.opaniuk@linaro.org>

Factorize embedded file list and rootfs generation for fvp, juno,
mediatek, qemu, qemu_v8 and rpi3.

busybox is madatory to all 'common' rootfs.
optee-client, xtestn helloworld are always embedded: moved to common.

Remove obsolete CFG_DEV_PATH.

Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>
Tested-by: Jerome Forissier <jerome.forissier@linaro.org> (HiKey)
Tested-by: Joakim Bech <joakim.bech@linaro.org> (RPi3, MTK8173)
Tested-by: Etienne Carriere <etienne.carriere@linaro.org> (qemu_v7/v8)

Besides the rpi3.mk file itself, this also contains the additional
kconfig files and flags used when building Linux and U-Boot.

A set of firmware images are also included.

Signed-off-by: Joakim Bech <joakim.bech@linaro.org>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: David Brown <david.brown@linaro.org>