image: Fix loop condition to avoid warning

GCC 5.1 starts warning for comparisons such as !a > 0, assuming that the
negation was meant to apply to the whole expression rather than just the
left operand.

Indeed the comparison in the FIT loadable code is confusingly written,
though it does end up doing the right thing. Rewrite the condition to be
more explicit, that is, iterate over strings until they're exhausted.

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