env: Use better name for variable in env_get_f()

The `nxt` variable actually points to the terminating null-byte of the
current env var, and the next env var is at `nxt + 1`, not `nxt`. So a
better name for this variable is `end`.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Simon Glass <sjg@chromium.org>