build - Separate out CFLAGS for kernel & module build

build - Separate out CFLAGS for kernel & module build

* Make COPTFLAGS apply to module builds in addition to kernel builds.

* Implement a new make variable, KCFLAGS, for specifying overrides for
  kernel and module builds.

* Fixup the module build infrastructure to use KCFLAGS, and get rid of
  infiltration from the more generic CFLAGS onto the kernel & modules
  build.