kernel: fix ccache variable sanitation for clang builds

* When using "?=", expansion doesn't happen right away, so the ccache
  variable is sanitized before KERNEL_CC gets expanded and no ccache
  is ever used for clang builds.

* With ":=", expansion happens immediately. So let's manually check
  if KERNEL_CC is already set somewhere else to determine whether or
  not we should set it ourselves using ":=".

Change-Id: I8a61767606a4f3d4c6ba88c68b10fd2e11783406