add generic lib/checksum.c

Add a generic (unoptimized) implementation of checksum.c in pure C
for use by all architectures that cannot be bother with implementing
their own version.

Based on microblaze code by Michal Simek <monstr@monstr.eu>

Cc: Michal Simek <monstr@monstr.eu>
Signed-off-by: Remis Lima Baima <remis.developer@googlemail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>