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)