tools/env: fix environment alignment tests for block devices

commit 183923d3 enforces that the
environment must start at an erase block boundary.

For block devices the sample fw_env.config does not mandate a erase block size
for block devices. A missing setting defaults to the full env size.

Depending on the environment location the alignment check now errors out for
perfectly legal settings.

Fix this by defaulting to the standard blocksize of 0x200 for environments
stored in a block device.
That keeps the fw_env.config files for block devices working even with that
new check.

Signed-off-by: Max Krummenacher <max.krummenacher@toradex.com>