kernel_imx: fix kernel build dependencies

	KERNEL_CONFIGURE (kernel_imx/.config) depends on _defconfig
		and is created by make xyz_defconfig

	TARGET_KERNEL_CONFIGURE (in product directory) is created
		by "install"ing kernel_imx/.config

	KERNEL_UIMAGE depends on KERNEL_CONFIGURE and is built by
		running "make uImage" in kernel_imx

	TARGET_PREBUILT_KERNEL (in product directory) is created
		by "install"ing from kernel_imx/arch/arm/boot/

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>