fng-install-u-boot: Install updated u-boot-environment and fw_env.conf

The u-boot environment comes packaged in the imx-boot.tar and is written
to disk. Location and size is specified by the packaged fw_enc.conf.
This is also stored in etc/shared for later usage through the tools.
Also add imx-boot.tar.gz to distro artifacts.

BCS 746-000600