lib/sha256: fix compile on some hosts

lib/sha256: fix compile on some hosts

Commit 2842c1c24269a05142802d25520e7cb9035e456c 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 <[email protected]>
Cc: Heiko Schocher <[email protected]>
Acked-by: Heiko Schocher<[email protected]>