package/environment-setup: new package

Install an helper script to setup a build environment based on
Buildroot. It's useful when a developer wants to use a Buildroot
generated SDK to build an external project.

Signed-off-by: Angelo Compagnucci <angelo@amarulasolutions.com>
Tested-by: Michael Trimarchi <michael@amarulasolutions.com>
Tested-by: Adam Duskett <aduskett@gmail.com>
Signed-off-by: Maury Anderson <maury.anderson@collins.com>
Signed-off-by: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>