tasks: kernel: Add support for Clang kernel builds

Add support for compiling kernels with Clang as intoduced on android-4.4

In order to compile with clang, you must set TARGET_KERNEL_CLANG_COMPILE.
If you wish to specify a specific clang version, also set TARGET_KERNEL_CLANG_VERSION,
but if that is not set, the build system will default to the latest version of clang
available in prebuilts/clang

Change-Id: I79dc1dca9758a930f2f5ba1754b5d8de114af900
Signed-off-by: Joe Maples <joe@frap129.org>