lib/sha256: fix compile on some hosts

Commit 2842c1c2 introduced lib/sha256 into
mkimage. Since then it will be compiled with HOSTCC which may produce errors
on some systems. Most BSD systems (like OS X for me) do not ship a
linux/string.h which will lead to take the U-Boot provided
include/linux/string.h in the end. This header howver is completely wrong
here. Just take the string.h if compiling with HOSTCC and linux/string.h when
not.

Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>
Cc: Heiko Schocher <hs@denx.de>
Acked-by: Heiko <Schocher&lt;hs@denx.de>