Too many people have CFLAGS set to support building userspace.
And now Kbuild picks up CFLAGS this caused troubles.

Although people should realise that setting CFLAGS has
a 'global' effect the impact on the kernel build is a suprise.
So change kbuild to pick up value from KCFLAGS that is
much less used.

When kbuild pick up a value it will warn like this:
Makefile:544: "WARNING: Appending $KCFLAGS (-O3) from environment to kernel $CFLAGS"
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Thomas Bächler <thomas@archlinux.org>
Cc: David Miller <davem@davemloft.net>
Cc: Ingo Molnar <mingo@redhat.com>