env: move env variables to a separate headers

This is a first part of the rework for [746-001456] Unify U-Boot Env for i.MX6 and i.MX8.

The goal of this MR is just a preparation step for further unification of environment between i.MX6 and i.MX8.

Background:

• originally, U-Boot environment for iMX6 boards was based on Edgehog environment and was just a proof of concept. Herewith the compatibility with U-Boot environment on iMX8 was not taken into account at all;
• as a result, boot logic/selection is hard-coded inside U-Boot environment on iMX6 boards itself;
• by contrast, external boot scripts are used on iMX8 board to implement this boot logic.

Current MR introduces just splits U-Boot environment variables in groups and isolates them in dedicated headers both on iMX6 and on iMX8 boards.

And subsequent MR will be in charge of:

• making existing "bootselect variables" for iMX8 boards work on iMX6 boards. In particular add support of boot scripts;
• preparation of boot scripts for iMX6 boards.