Suppress echoing of commands when using "make -s", so that make -s
does indeed have the effect one would expect.

Add a quiet mode, which will print not the entire command but only
one line per rule. To turn it on, use

	make KBUILD_VERBOSE=0 vmlinux/whatever

or set KBUILD_VERBOSE=0 in your environment.

For now, the verbose mode is default, which gives you the old behavior
of printing all commands.

The output in quiet mode is based on what Keith Owens' kbuild-2.5 does,
I like, I did not want to invent yet another output format.