kernel.mk: set KERNEL_OUT properly

It was hardcoded to $(ANDROID_BUILD_TOP)/$(TARGET_OUT_INTERMEDIATES)/KERNEL_OBJ
which wouldn't exist if OUT_DIR_COMMON_BASE was set to use another dir
(e.g. ramdisk)

Fix it so that it now points to $(TARGET_OUT_INTERMEDIATES)/KERNEL_OBJ
if OUT_DIR is not out

Change-Id: I1bf767d86548e41270d9cbb8f0c00512708501c5