Makefile: generate a Makefile wrapper in $(O)

If building out-of-tree, add a Makefile wrapper that calls-out to the real
Makefile with proper args.

Avoids having to pass -C and O= every time we call make.

This is highly inspired from how the Linux kernel does it, and portions of
it have been used. We can't use exactly the same implementation as the
kernel does, because:

 - the script writing the wrapper has been expunged of the few lines
   that were too kernel-related: in buildroot we do not need the version
   string in the wrapper, and we do not have a patchlevel version;

 - "in-tree build" does not have the same meaning for the kernel and for
   buildroot: for the kernel, $(O) point to the $(TOPDIR), while for
   buildroot $(O) points to $(TOPDIR)/output.

For more complete explanations, see:
  http://lists.busybox.net/pipermail/buildroot/2010-September/037815.html



[Peter: minor tweaks]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998 at anciens.enib.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>