tools: Honour the O= flag when tool build called from a higher Makefile

Honour the O= flag that was passed to a higher level Makefile and then passed
down as part of a tool build.

To make this work, the top-level Makefile passes the original O= flag and
subdir=tools to the tools/Makefile, and that in turn passes
subdir=$(O)/$(subdir)/foodir when building tool foo in directory
$(O)/$(subdir)/foodir (where the intervening slashes aren't added if an
element is missing).

For example, take perf.  This is found in tools/perf/.  Assume we're building
into directory ~/zebra/, so we pass O=~/zebra to make.  Dependening on where
we run the build from, we see:

	make run in dir		$(OUTPUT) dir
	=======================	==================
	linux			~/zebra/tools/perf/
	linux/tools		~/zebra/perf/
	linux/tools/perf	~/zebra/

and if O= is not set, we get:

	make run in dir		$(OUTPUT) dir
	=======================	==================
	linux			linux/tools/perf/
	linux/tools		linux/tools/perf/
	linux/tools/perf	linux/tools/perf/

The output directories are created by the descend function if they don't
already exist.

Signed-off-by: David Howells <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Paul Mackerras <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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