include: configs: am62x_evm: Add .env file for Am62x

Use .env file for setting board related environment variables,
in place of am62x_evm.h file. Except for BOOTENV settings, as
config_distro_boot.env file doesn't exist.

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