tools/env: Fix environment size and CRC on 64-bit hosts

On architectures where 'long' is 64 bit, the u-boot environment
as seen by the fw_env tools was missing 4 bytes.
This patch fixes getenvsize(), and thus also ensures that the
environment's CRC32 checksum is calculated correctly.

Signed-off-by: Dominic Sacré <dominic.sacre@gmx.de>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Tom Rini <trini@ti.com>