Fix build issue with string.h and linux/string.h

This commit:
commit 338cc038
Author: Wolfgang Denk <wd@denx.de>
Date:   Fri Jun 6 14:28:14 2008 +0200

    tools/mkimage: fix compiler warnings on some systems.

Broke building on some systems, because the host's string.h was interfering
with u-boot's linux/string.h.  It doesn't look like we need the u-boot one if
we're building for the host, so now we only include when building inside
u-boot.

Signed-off-by: Andy Fleming <afleming@freescale.com>