new tool mkenvimage: generates an env image from an arbitrary config file

This tool takes a key=value configuration file (same as would a `printenv' show)
and generates the corresponding environment image, ready to be flashed.

use case: flash the environment with an external tool

Signed-off-by: David Wagner <david.wagner@free-electrons.com>
Acked-by; Mike Frysinger <vapier@gentoo.org>
Tested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>