tools/env: allow negative offsets

A negative value for the offset is treated as a backwards offset for
from the end of the device/partition for block devices. This aligns
the behavior of the config file with the syntax of CONFIG_ENV_OFFSET
where the functionality has been introduced with
commit 5c088ee8 ("env_mmc: allow negative CONFIG_ENV_OFFSET").

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>