include: environment: ti: Use .env for environment variables

Add K3 common environment variables to .env. We retain the old-style C
environment .h files to maintain compatibility with other K3 boards that
have not moved to using .env yet.

Signed-off-by: Neha Malcom Francis <n-francis@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>