hikey: do not build kernel modules

We currently do not copy any kernel module into our root FS, they are
not needed. Therefore, drop the "modules" target when building the
kernel to save compile time.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>