Fold kernel-cflags-finder into the Makefiles

[geofft]

(Previous attempt in #185 - deferred in #188)

Now that we're running cargo from inside a kernel build Makefile instead of standalone (#177), we shouldn't need to run another kbuild make to figure out what the flags are. We should just be able to pick that up from the current Makefile variables and send it to build.rs via an environment variable or something.

[alex]

The problem here is you want CC=gcc in the outside makefile and CC=clang for the cflag-finder makefile.

We need to be able to drive the outside makefile with CC=clang to make this work