libfdt

It has been observed that fit_check_format() will fail when passed a
corrupt FIT image.  This was tracked down to _fdt_string_eq():
return (strlen(p) == len) && (memcmp(p, s, len) == 0);

In the case of a corrupt FIT image one can't depend on 'p' being NULL
terminated.  I changed it to use strnlen() to fix the issue.

Signed-off-by: Tom Rini <trini@ti.com>