env: Fix env_get() when returning empty string using env_get_f()

The env_get_f() function returns -1 on failure. Returning 0 means that
the variable exists, and is empty string.

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