tools build: Add fixdep dependency helper

For dependency tracking we currently use targets that fall out of the
gcc -MD command. We store this info in the .cmd file and include as
makefile during the build.

This format put object as target and all the c and header files as
dependencies, like:

  util/abspath.o: util/abspath.c /usr/include/stdc-predef.h util/cache.h \
   /usr/include/bits/wordsize.h /usr/include/gnu/stubs.h \
   ...

If any of those dependency header files (krava.h below) is removed the
build fails on:

  make[1]: *** No rule to make target 'krava.h', needed by 'inc.o'.  Stop.

This patch adds fixdep helper, that is used by kbuild to alter the shape
of the object dependencies like:

  source_util/abspath.o := util/abspath.c

  deps_util/abspath.o := \
    /usr/include/stdc-predef.h \
    util/cache.h \
    ...

  util/abspath.o: $(deps_util/abspath.o)

  $(deps_util/abspath.o):

With this format the header removal won't make the build fail, because
it'll be picked up by the last empty target defined for each header.

As previously mentioned the fixdep tool is taken from kbuild. It's not
complete backport, only the part that alters the standard dependency
info was taken, the part that adds the CONFIG_* dependency logic will be
probably taken later on.

Signed-off-by: Jiri Olsa <[email protected]>
Cc: David Ahern <[email protected]>
Cc: Kai Germaschewski <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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