env: Use string pointer instead of indexes in env_get_f()

Since we no longer use env_get_char() to access n-th character of
linearized environment data, but rather access the arrays themselves, we
can convert the iteration to use string pointers instead of position
indexes.

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