tools/env: return with error if redundant environments have unequal size

For double buffering to work, the target buffer must always be big
enough to hold all data. This can only be ensured if buffers are of
equal size, otherwise one must be smaller and we risk data loss
when copying from the bigger to the smaller buffer.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Andreas Fenkart <andreas.fenkart@digitalstrom.com>
Reviewed-by: Tom Rini <trini@konsulko.com>