Add crc32sum.c
Add crc32sum - a tool for calculating crc32 check sums in the same manner as u-boot.

Signed-off-by: Jon Seymour <jon@ninjablocks.com>
Build x86_64/Linux versions of two tools - mkenvimage and crc32sum.

Signed-off-by: Jon Seymour <jon@ninjablocks.com>