Suspected Spam: Do not open attachements![PATCH 4/6] tools/env: flash_write_buf: enforce offset to be start of environment

This allows to take advantage of the environment being block aligned.
This is not a new constraint. Writes always start at the begin of the
environment, since the header with CRC/length as there.
Every environment modification requires updating the header

Signed-off-by: Andreas Fenkart <andreas.fenkart@digitalstrom.com>