include: configs: am62ax: Change to using .env

Move to using .env file for setting up environment variables for am62ax.

This patch depends on
https://lore.kernel.org/all/20230315052745.110502-1-n-francis@ti.com/



Signed-off-by: Nikhil M Jain <n-jain1@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>