riscv: Fix range looking for kernel image memblock

When looking for the memblock where the kernel lives, we should check
that the memory range associated to the memblock entirely comprises the
kernel image and not only intersects with it.

Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>