kernel: Add TARGET_FORCE_PREBUILT_KERNEL flag

* TARGET_FORCE_PREBUILT_KERNEL should be useful when you don't have ready kernel sources (OEM releasing broken sources or releasing them lately)
* It lets you use kernel sources for userspace generated headers (generated_kernel_headers) while keeping using prebuilt kernel
* Keep in mind that generated kernel headers may not align with the ABI of kernel you're including
* This commit also partially revert commit a836d796, where TARGET_KERNEL_SOURCE would have been unset if TARGET_PREBUILT_KERNEL was defined (we already check in kernel.mk if kernel sources are present anyway)

Change-Id: I84acae890dabb5b9dbd48143ad58f6420c0cef75