Rename existing environment variables according to the bootstd doc.
Renamed variables are fdto_addr, bootenv_addr, fdt_addr.

Signed-off-by: Benjamin Hahn <B.Hahn@phytec.de>
Signed-off-by: Daniel Schultz <d.schultz@phytec.de>
Reviewed-by: Wadim Egorov <w.egorov@phytec.de>

The env scripts for loading and applying overlays are identical for many
PHYTEC Boards. Create a common env that can be included.

The env variables bootenv_addr and fdto_addr are board specific and need
to be set in the board specific file. The env variable get_cmd also
needs to be set in board specific files and can be set to tftp or dhcp.

Signed-off-by: Benjamin Hahn <B.Hahn@phytec.de>
Signed-off-by: Daniel Schultz <d.schultz@phytec.de>
Reviewed-by: Teresa Remmet <t.remmet@phytec.de>
Reviewed-by: Wadim Egorov <w.egorov@phytec.de>