kcov: allow more fine-grained coverage instrumentation

For more targeted fuzzing, it's better to disable kernel-wide
instrumentation and instead enable it on a per-subsystem basis.  This
follows the pattern of UBSAN and allows you to compile in the kcov
driver without instrumenting the whole kernel.

To instrument a part of the kernel, you can use either

    # for a single file in the current directory
    KCOV_INSTRUMENT_filename.o := y

or

    # for all the files in the current directory (excluding subdirectories)
    KCOV_INSTRUMENT := y

or

    # (same as above)
    ccflags-y += $(CFLAGS_KCOV)

or

    # for all the files in the current directory (including subdirectories)
    subdir-ccflags-y += $(CFLAGS_KCOV)

Link: http://lkml.kernel.org/r/1464008380-11405-1-git-send-email-vegard.nossum@oracle.com


Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Quentin Casasnovas <quentin.casasnovas@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>