greybus: build: android: avoid building for targets that don't have kernel binary

For sdk related targets, the greybus build will error out due to missing
kernel dependency.  Let's avoid those targets by checking TARGET_NO_KERNEL.
Signed-off-by: Vishal Bhoj <vishal.bhoj@linaro.org>
Signed-off-by: Michael Scott <michael.scott@linaro.org>
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>

drivers/staging/greybus/Android.mk

 .PHONY: build-greybus
 
+ifneq ($(TARGET_NO_KERNEL), true)
 $(PRODUCT_OUT)/ramdisk.img: build-greybus
+endif
 
 GREYBUS_MODULE_OUT_PATH := $(PRODUCT_OUT)/root/lib/modules