Cleanup the list of headers we include here. For the tools build we only
need to exclude <cyclic.h> as that's used by the target build for the
prototype for schedule(), and we don't need to get that via
<watchdog.h>. We can also make use of our <string.h> intentionally
existing as a redirection to <linux/string.h> to reduce ifdef'd lines.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@konsulko.com>