ARM: imx: cm_fx6: env: use standard variables

In preparation for supporting the distro boot command, introduce the
standard variables for specifying load addresses, which are documented
in README and doc/README.distro, and replace the custom variables
used so far with them.

Since the current address layout disregards an address for an initramfs,
also switch to the load addresses used and proven by other imx6 boards
(e.g. the wandboard and nitrogen6x), instead of going on with our own
way.

Signed-off-by: Christopher Spinrath <christopher.spinrath@rwth-aachen.de>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>