fw_env: Add missing write failure check

If flash_write fails, whilst we propagate this up to our caller, we need
to avoid swapping in the new file (if we're on a filesystem) in this
case.

Fixes: dbc34323796b ("tools: env: Implement atomic replace for filesystem")
Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
1 file changed